This article is from the Car Audio FAQ, by Ian D. Bjorhovde (firstname.lastname@example.org) with numerous contributions by others.
Many factory head units these days have the ability to control a
remotely mounted cd changer. Generally, the head will have a button
labelled "CD" to switch sources to the external changer. In this mode
either the radio preset buttons and/or the tuner up/down buttons will
control which CD and/or track is playing. Check your car's manual to
make sure your head can control a changer and how the buttons work.
Once you know your head can control a changer, you wonder "What kind of
changer will work with my factory head unit?" Of course, the one the
dealer wants to sell you will work. However, the dealer makes lots of
money selling you a changer, and there are often other after-market
solutions, usually involving an adapter cable and a name-brand changer.
The dealer will tell you that their solution is better and that's why it
costs so much more (often more than twice as much as an aftermarket
The car manufacturers are constantly changing the interfaces between
their heads and changers, in an effort to get you to buy their solution.
However, the after-market is constantly reverse-engineering the
interfaces and providing alternative solutions for the cost-conscious
Two companies that make such adapters are Precision Interface
Electronics (or PIE, <http://www.pie.net>) and Peripheral Interface
Components (<http://www.stinger-aamp.com/peripheral/s-ind.htm>). Check
their web sites to see if there's an adapter for your car's factory
head. They also list which changer(s) will work with their adapters.
For example, many of Honda's late-model heads were made for them by
Alpine, so the OEM changer you'd pay your friendly Honda dealer ~$700
for is essentially the same as Alpine's changers. The only difference
is the interface wiring, where they swapped two pins, specifically so
you'd have to get it from the dealer. (If you're interested in the
after-market adapters for this head simply swap the pins back, so you
can use the regular Alpine changer, which can be bought for ~$300.
Once you know which adapter/changer combo will work, you can get it from
your local car audio dealer or favorite mail order place. The advantage
of getting it from a local dealer is that they'll be able to install it
for you. However, if you have the time and are at all mechanically
inclined, you should readily be able to install it yourself.