This is the unit I have been using for mine; the Monster Icruze. It attaches to the factory CD-Changer cable and allows the ipod to be controlled with factory settings.
Monster iCruze
Great thing about them is that you can pick them up for under $20 on ebay.
Sound Quality is excellent, the only drawback is that it can only play songs that are assigned to a playlist, and it can only play 6 playlists at a time.