If you dont want to spend loads of money, time and effort installing an ipod or yampp into your cars, simply get one of those cool tape thingys (lol i dunno what theyre called but i got one). You simply plug the jack into the headphone/output of your mp3 device, and put the tape into the player! I know its a crummy idea, but it costs nothing compared to installing nd wiring, updating nd maintaning one in your car! besides, what if it gets stolen?! If you do it my way, you can take your ipod about with you wherever you go.
There is also another way. Its called the Griffin iTrip FM Transmitter, and when plugged in (obviously) will broadcast your music enough for your car radio to pick up. This tool is illegal in England (classed as making your own radio station, which isnt allowed), and initaily only for the ipod, but all you do is plug it into the jack socket. So basically you can plug it into anything with a jack socket!