yeah, theres things that plug into the i wanna say its called a cigarette lighter? but ive never seen it used for that.. anyway, you can buy them at best buy for about 40/50 bucks, and you turn your radio to like 88.9 while its plugged in and it works the same way as the AUX imput
If the car has a tape deck???? you can get an attachment that goes in to the slot, you then plug the lead into the iPhone's earphone jack and it then plays from the iPhone, I play a CD unit that way, try a radio type shop for it.