You simply need an FM transmitter.
You plug the lead from the transmitter into your MP3 player's headphone socket and press the buttons on the transmitter to choose a frequency which isn't used on your car radio. (In practice that means something either at the very top end of your car's VHF reception range, or at the very bottom of it). Then your car radio will simply see your MP3 player as an extra radio station at that frequency.
Such transmitters are widely available, including in most big supermarkets. However (unless your car hasn't got a cigar lighter socket) you should take care to choose one which plugs into the cigar lighter, rather than needing batteries to power it.
I've got one of these
http://asda.scene7.co...89244?hei=400&wid=400
but, annoyingly Asda no longer stock it (with their replacement product needing batteries):
http://direct.asda.co...mId=xtaqqlxt&istBid=t
Similarly, Tesco seem to have opted for a battery-powered model:
http://direct.tesco.c...&utm_content=209-0190
The best deal I can find on a transmitter powered from the cigar lighter (other than from suspect overseas sellers) is from Play.com:
http://www.play.com/M...t:3516493|prd:3516493
Chris