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The first component in the music chain is the head unit, often referred to simply as the radio though the head unit does much more than just tune stations. The head unit you choose for your system depends on your budget and your current collection of music. There are two main categories of head units available. These are cassette players and compact disc or CD receivers. The first decision to make when choosing a head unit is whether or not to replace the factory head unit. The quality of original equipment manufacturer (OEM, those that come with your vehicle from the factory) head units has increased to the point where, if features are ignored, the unit will be acceptable for most listener's tastes. If the automobile is lacking an adequate deck or if the automobile is not a recent model, a replacement deck is needed. Keep in mind that factory decks are usually easier to use with larger and fewer buttons and of course look like they belong in the dash (because they do). Also, a CD changer can be added to a factory deck (or deck lacking CD changer controls) through the FM antenna using what is known as a modulator.
Your factory head unit may also have inputs and controls for a CD changer which was not installed at the time of purchase. These head units can either have a factory CD changer added later or certain aftermarket changers added with special adapters available through a few select companies such as PIE (Perfect Interface Electronics), Blitzsafe and others. Typically it will be cheaper to have an aftermarket changer installed with an adapter than to buy a factory unit. The advantage over an FM modulated changer will be higher sound quality and one less controller in the dash. Factory units may also have controls built into the steering wheel which can be difficult to replicate with an aftermarket head unit. However PAC makes a few units that can rememedy this for many models.
Also some cars have additional functions built into the head unit that are not audio related that can prohibit the removal of the factory deck. In this case an educated installer can relocate the factory head unit to retain these features while still adding a factory head unit. Over time aftermarket adapters are often created that mimic these functions so radio relocation is not necessary.
Manufacturers do not want aftermarket head units in their cars because this cuts into their profits on premium sound systems they want to sell. These additional functions built into the radio are part of this plan. They have also changed the size of head units from the standard DIN (single height head unit) to 1 ½ DIN, double DIN, oval shaped, and split units such as the older GM pickups with the radio controls near the steering column and the tape/CD/equalizer in the middle dash. Manufacturers such as Metra and Scosche offer dash kits that will facilitate the addition of standard DIN radios in these vehicles. Some aftermarket head unit manufacturers, such as Pioneer, also offer non-standard size units to fit the more popular vehicles with 1 ½ and double DIN.
If you have a classic automobile with a vintage radio shaft pattern and do not wish to cut into the dash to install a DIN unit you should contact a specialty radio manufacturer such as Custom Autosound or Absolute Classics that supplies these style radios.
If you decide you are able and desire to use an aftermarket head unit then the next step is to decide if you want a tape or CD unit. If you have an extensive tape collection you may choose to buy an in dash cassette player. You should also decide if you will wish to add a CD changer in the future. If you do then you will want to purchase a unit with built in CD changer controls. If you do not purchase this type of cassette player then your only choice for an add on CD changer will be an FM modulated unit with a separate control panel and lower sound quality (due to the interface type).

Cassette Player

Typically a decent cassette player with CD changer controls will start in the $100 range. The price increases up to $300-400+ for top of the line units. When shopping for a cassette receiver there are some basics to look for. The most important is the company’s reputation for building quality product. Purchasing a tape player at a flea market is usually not a wise decision. Security is also an important factor that should be considered. Most decent tape players will have a detachable faceplate. I wouldn't recommend purchasing one that did not as these are usually the lowest models offered by a company. Of course with cassettes being such an old technology you will probably not find many choices now or in the future. You can search for used models if you really need a high quality cassette player.
The last major item to consider is whether the unit will fit in your dash. In addition to depth considerations, the face style is also important. Models with flat faces look more modern but aren't much good to you if you have a car with a shaft style opening that you don't want to cut. You will also want to make sure that the depth of the unit will allow installation in your dash. Look for a control layout that you are comfortable with. Usually a simple layout is better for finding buttons quickly. Also, a crowded and busy display looks great in the showroom but can be confusing as well as dangerous when driving down the road at highway speeds. Beyond these basics you need only worry about which features you wish to have. The more you want, the more you will likely have to pay.

Some of the more common features available in most major brands are:

Auto Reverse: The tape will turn over and begin playing the other side of the tape when the end of one side is reached.
Changer Controls: This is the ability of a receiver to control an external CD, MiniDisc or MP3 player through its front panel controls. With this option a digital source may be used in conjunction with an in dash cassette receiver.
Dolby Noise Reduction (A,B,C,S): Dolby noise reduction was invented by Dolby Laboratories and is available in varying degrees of noise reduction with A being the least and S being the most. The most common method is Dolby B, found on most cassette tapes.
Dual Illumination: This is a receiver’s ability to change the color of its display between two or more colors, usually amber and green. This is a nice feature to have if you go through a lot of vehicles and are never sure of your next vehicles dash color.
Frequency Response: This is the portion of the audio spectrum a piece of equipment can produce. The average human can hear sounds in the range of 20Hz-20kHz. Most good tape players can produce from about 30Hz-18kHz.
Full Logic Controls: This allows the receiver to have electronic control rather than manual control of the cassette unit’s functions. It also understands fast forward means move forward on the tape, regardless of which side is playing (on auto reverse heads). I personally find this feature extremely important as I have hit fast forward many times when the deck was in auto reverse mode only to have the tape rewound because the unit didn’t understand the difference.
Line Output Voltage: This is the output voltage that a deck can deliver, in volts, through its pre-amp line outputs. The higher than number the less chance that noise will be delivered to the next piece of equipment in the chain. Most lower to mid priced decks have an output of 500 mV (0.5 volts) while some higher end units have 4 volts or more.
Loudness Control: This is a feature that boosts the lower and sometimes the very upper frequencies to compensate for the human ear's insensitivity at low volume.
Metal Tape: This is an extra feature that will better optimize the sound of any metal type cassettes you may have. Metal tapes are not very common and this feature is rarely used by most.
Music Search/Multi-Music Scan: Music search is the ability of a player to sense a break in program material it searches for in either fast forward or rewind and begins playback when it finds this break. Multi-music scan allows a player to search for a specified number of breaks and stop on the specified break. This is a very nice feature if you know that the song you want to listen to is three songs ahead.
Power Output: This is the amount of power, in watts, that your receiver should be able to deliver to the speakers. The number printed on the face of the receiver is usually a large exaggeration. Most receivers actually have between three and fifteen watts RMS per channel. Keep in mind that the sound will become quite distorted at this level because of the size and current limitations of your receiver. The best sound is always found in dedicated external amplifiers.
Pre-amp Outputs (RCA jacks): These are the most common connectors used for external power amplifiers. If you plan to add amplifiers to your system I strongly suggest that you purchase a receiver with these outputs. More than one set of outputs are available on some of the better units and are handy for retaining front/rear/subwoofer fading capabilities.
Remote Control: Most people laugh at the idea of having a remote control for an in dash receiver but it is actually much safer to use when on the road. Models are available in large TV sized remotes, handheld remotes, credit card remotes and even steering wheel mounted remotes.
Station Presets: These are the areas of memory your receiver stores radio station frequencies. The more the better here. Some units even allow you to assign names to the various presets.
Seek/Scan: These functions are used to seek for the next station that is available and scan through your preset stations respectively.
Tuner Sensitivity: This is the ability of the tuner in your receiver to pull in weak stations. It is measured in dbf and the lower the number the better. Eight is about the best you will likely see and thirteen is about average.

CD Player

The criteria to look for when shopping for a CD player are similar to those when shopping for a cassette player. Again, the most important is the company’s reputation for building quality product. Most decent CD players will have a detachable faceplate. I wouldn't recommend purchasing one that did not as these are usually the lowest models offered by a company. The last major item to consider is whether the unit will fit in your dash in terms of depth. Look for a control layout that you are comfortable with. Usually a simple layout is better for finding buttons quickly. Also a crowded and busy display looks great in the showroom but can be confusing as well as dangerous when driving down the road at highway speeds. Beyond these basics you need only worry about which features you wish to have. The more you want the more you will likely have to pay.

Some of the more common features available in most major brands are:

Audible Search: With this feature you will be able to hear the music that you are searching through which can be a very desirable feature.
Changer Controls: This is the ability of a receiver to control an external CD, MiniDisc or MP3 player through its front panel controls. With this option a digital source may be used in conjunction with an in dash cassette or CD receiver.
Dual Illumination: This is a receiver’s ability to change the color of its display between two or more colors, usually amber and green. This is a nice feature to have if you go through a lot of vehicles and are never sure of your next vehicles dash color.
Frequency Response: This is the portion of the audio spectrum a piece of equipment can produce. The average human can hear sounds in the range of 20Hz-20kHz. Most good CD players can produce this entire range.
Line Output Voltage: This is the output voltage that a deck can deliver, in volts, through its pre-amp line outputs. The higher than number the less chance that noise will be delivered to the next piece of equipment in the chain. Most lower to mid priced decks have an output of 500 mV (0.5 volts) while some higher end units have 4 volts or more.
Loudness Control: This is a feature that boosts the lower and sometimes the very upper frequencies to compensate for the human ear's insensitivity at low volume.
MP3 capability: Some of the newer CD head units have the ability to play MP3 encoded discs. This can be a real benefit if you have a CD burner and a collection of MP3s on your computer as it eliminates the need to have additional equipment to play your MP3 collection in your automobile.
Power Output: This is the amount of power, in watts, that your receiver should be able to deliver to the speakers. The number printed on the face of the receiver is usually a large exaggeration. Most receivers actually have between three and fifteen watts RMS per channel. Keep in mind that the sound will become quite distorted at this level because of the size and current limitations of your receiver. The best sound is always found in dedicated external amplifiers.
Pre-amp Outputs (RCA jacks): These are the most common connectors used for external power amplifiers. If you plan to add amplifiers to your system I strongly suggest that you purchase a receiver with these outputs. More than one set of outputs are available on some of the better units and are handy for retaining front/rear/subwoofer fading capabilities.Remote Control: Most people laugh at the idea of having a remote control for an in dash receiver but it is actually much safer to use when on the road. Models are available in large TV sized remotes, handheld remotes, credit card remotes and even steering wheel mounted remotes.
Skip Protection: Some receivers have a built in memory buffer, usually between three and ten seconds, that will continue to play the music skip free even when the player is actually mis-tracking. This is a great feature if you drive on a lot of bumpy roads but is an uncommon feature usually reserved for higher end changers and MD units.
Station Presets: These are the areas of memory in which your receiver stores radio station frequencies. The more the better here. Some units even allow you to assign names to the various presets.
Seek/Scan: These functions are used to seek for the next station that is available and scan through your preset stations respectively.
Tuner Sensitivity: This is the ability of the tuner in your receiver to pull in weak stations. It is measured in dbf and the lower the number the better. Eight is about the best you will likely see and thirteen is about average.