There are possibly two ways you can do this. You can use an FM transmitter to send the signal to an unused FM station in your receiver or you can use the Line In or Audio Input that may be in the back of you Radio, depending on how old it is. The FM transmitter is similar to the MP3 player type transmitter advertised Online or at Best Buy or Circuit City.

A third way would be to use a cassette adapter (if your car stereo can play cassettes). I've tried the FM transmitters with my iPod and found they're not as reliable as I'd like them to be. OTOH, cheaper cassette adapters are rather noisy. But I can always turn up the radio.* :-)

* Don't try this at home. :-)