In my quest to find the ultimate in-car MP3 player, I ended up making adapters for my iPod. I made one for a Honda del Sol, and one for an Audi A4.
The iPod adapter for the del Sol was easy, as the after-market stereo had a line-in. I just made a pretty input jack. Check it out at http://www.mikeyp.com/delsolipod.
The Audi adapter was more challenging, as there was no line-in, nor did I want to string cables through the car's interior. For the line-in, I bought an adapter from http://www.autotoys.com that plugs in to the CD-Changer input. For the cabling, believe it or not, the 2001 A4 has a pre-wired Cat5 cable that runs from the center console to a DB25 the trunk! The result is here: http://www.mikeyp.com/a4ipod.