I've found that the old school method works best for me. If you have a tape deck, get one of those 'cd player adapters' that has a casette adapter and a headphone jack. Plug it into the ipod, and pop the casette adapter in to the tape deck.
If you don't have a tape deck in the car, you may need to use a radio transmitter (as mentioned by the other person).
Radio is prone to interfearance (why I like the tape deck adapter better) but it's wireless.