A CD changer for your car stereo gives you the chance to load up multiple discs and cycle through them without having to physically handle CDs while you're driving. CD changers are often mounted in the trunk of a car and accept a cartridge with the CDs in place; the audio is piped up to the head unit for control from the driver's -- or passenger's -- seat.
1Inputs and Connections: Whether or not you're able to use a CD changer depends on if you have the necessary inputs on your car's head unit. The fast way to check is to look to see if you have an AUX input button on the radio. You can use the auxiliary input to bring in the audio from the CD changer. Some stereos are designed with the inputs and controls for a CD changer; look for a Changer button on the stereo to see if that's the case for yours. If you're not sure, check the manual for your stereo and your car to examine the wiring and connection situation. Some cars are designed with the wires already in place for a CD changer -- in case the buyer ordered it from the factory that way -- and so only take a few connections to make things work. Research at the outset will help in the long run.
2Location: Where you put the changer is an important choice in the process. If you put it in the trunk -- mounted to the top of the trunk just inside the lid, it's not visible to people walking by your car and it doesn't get in the way of most uses. However, you'll have to open the trunk every time you want to swap out CD cartridges and you'll need to figure out how to get the wires from the back of your car to the front. If you have a hatchback or a truck, trunk mounting isn't an option, but you might be able to find space in the back seat, under a seat or in a center console. The top considerations are to find a place to which you can easily run the wires and that won't get in the way of normal vehicle operation.
3Factory Connections, Adapters and Modulators: Depending on the connections you found, you might need to use a vehicle-specific connector or an adapter to get the CD player working with your head unit. If you have an auxiliary or CD changer input for your deck you'll want to find either a factory CD changer that's compatible with your car or an adapter to make it compatible. If you don't have an auxiliary input, you can still use a CD changer by hooking it in to the FM antenna with a modulator. One of your FM stations will be lost to the cause, but you'll be able to listen to copious amounts of CD music in exchange.
4Safety and Wiring: Whenever working on electrical systems in your car, disconnect the negative battery cable from the battery and tie it back so it won't slip back into contact with the terminal. The 12 volt power system of a car could deliver enough electricity to you to hurt you. Car stereo wiring may look like the guts of C-3PO splayed out for all to see, but the color coding makes it simple enough to figure out (see link in Resources). Take your time, make solid connections and you'll be rocking out to your favorite tunes in the middle of traffic in no time.