Sounds like a very difficult task... for me at least.
Just FYI, they do sell adapters for most factory stereo cd changer inputs to connect any 1/8" audio out. Also, they make them specifically for Ipods so that the controls on the HU are more specific to the ipod.
I know I found a site real easy for my honda's factory stereo, but I ended up getting a new stereo anyway. The adapter would have been $30 online.