This article is from the Car Audio FAQ, by Ian D. Bjorhovde (email@example.com) with numerous contributions by others.
Almost all manufacturers offer an FM modulator for their changers. As
with all equipment, some are good and some are not. A person thinking
about using an FM modulator must take into consideration that the sound
quality will only be as good as the tuner in your head unit. Also, FM
is limited in its frequency response. There is usually a noticeable
loss of the high frequencies, due to the nature of transmitting via FM.
If you do not want to use an FM Modulated CD changer, some
manufacturers make controllers for changers that feature line-level
(RCA) outputs. This allows you to connect the changer directly to an
amplifier, bypassing the stock system altogether. Some models offer
line-level inputs, allowing you to connect the stock system to the
changer (so you can continue to use your radio/tape). Clarion, Sony,
and Kenwood make such units.
Your third option is to use the aftermarket changer that corresponds to
the stock unit in your car. Not all cars have this option, but it is
becoming easier. (see Section 3.25).