the easiest way to do this would be to buy an FM transmitter. i have a belkin one which you can set to any station, and it works like a dream. unless you're going for insane audio quality, you won't notice that its FM signal.. you basically plug the transmitter into your mp3 player and then tune both your car radio and the transmitter to the same frequency and bam, there's your media player running through your stereo. the other way i used to do it was i had an aftermarket head unit installed in the car, which had an auxillary input, for plugging other media sources into the stereo. this is a better quality than FM transmitter, but to be honest you really cant tell... do you have an ipod? if you do, there are a number of different head units you can buy which will interface directly with the head unit so you can change songs on the unit as well as see track information on the screen... the final way is by direct USB, but you'd need a head unit that supported that and they are pretty rare... mine is all linked through my car PC, which is another alternative altogether and alot more work :P