You also might check out Make magazine - a mag that helps people modify things in non-conventional ways. The current issue has an iPod integration in it, I think, and I know they have some electronic car mods in it.
http://www.makezine.com.
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I found iCarPlay Wireless FM to be the best solution so far. This will charge the ipod and has a FM transmitter.
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the downside is wireless FM has really bad sound quality. and its pretty annoying trying to change the frequency in a city where the airwaves are congested.