if you have a tape player in your car, i've seen people buy these casette-looking things and they put in the tape player, and then there's a cord that plugs into where your headphones go and it plays your ipod over the speakers.
if you dont have a tape player, i've also seen these things you put in your lighter and you chose an empty radio station and it just plays your music over the speakers
i know that's a really shitty explanation of the two, and it probably made no sence, but if you go to target or radioshack or something, they're usually right next to where the ipods are. i think the cig. lighter one is pretty expensive.