Well, if you have an iPod or are willing to get one, your best bet (or a good bet, anyway) would probably be the
Harman Kardon Drive + Play. You can plug that either to an aux input on your car stereo if you have one, or you can plug your antenna cable into its control box and connect the outgoing antenna cable from that control box to your stereo.
Now, I should say I've never used it, but I see no reason for it not to work as advertised, and it looks very slick indeed. You'd have to do some installation work but I doubt if it is that hard.
Not sure what other options you have, except for instance getting a short-range FM radio transmitter for your MP3 player and using that. The kit above does allow you to integrate it much more prettily though.