I can't make a guess at the pinouts of the changer without hacking into it, I suspect. That said, I thought I saw something on this site about a company that sells a little in-line adaptor for doing just what you describe.

I could be on crack, though. Try searching for ipod.

In any case, my stock stereo is sitting on my kitchen table right now. There's a picture of the cable here: here. Let me know if you want something more specific.