http://www.crutchfield.com/App/Produ...MMOD01&tp=1701
These are nice. They are like the FM transmitters that plug into your 12V lighter, but hard wired into the antenna, for much less interference. But its hard wired so stays in one vehicle. It sounds like you want to be able to use in in different vehicles, so I would pick up something like this
http://www.crutchfield.com/App/Produ...21IP163&tp=120
You can get similar ones all over the place. Basically it plugs into your IPOD and then sends the signal over a predetermined FM station, you just tune into the station. You will get a little interference, but usually not too bad.