Try these guys:
http://pac-audio.com/
I found them from another link in this section of the forums, but I used the device in my 98 Deville, it works 99% flawlessly. Occasionally though when I turn the car on it does not unpause the ipod. So I have to cycle though sources once or twice, sometimes 3 times and it'll play again.
The support says my head unit in the Deville is too 'dumb' and only can send Skip / Back commands to the ipod, but somehow when I turn off the car it does pause.
Overall I am very happy though, much rather have the ipod with more music vs the old 12 disc changer in the trunk (that was broken anyway)
One more thing, I have learned I need to unplug the ipod if I am not going to start the car for 5 days. I went on vacation and left the ipod plugged in and came home to a dead battery. When the car is off there is still power to the ipod.