to fit into a factory audio system, i would get the factory CD changer that plays MP3 files on CD. i use to have an aftermarket CD changer with a i-BUS adaptor, and it worked well, but was slow on changing tracks; the factory unit is much better and in the end costs about the same if you know where to shop (aka, not at the dealer).
if you have an ipod, i would consider an ipod integration unit rather than a cd changer.