And this cable has a headphone jack on one end and RCA jacks on the other:

Click on the photo and it will whisk you away to the mp3car.com store where you can purchase said cable.
I think you know where the headphone jack goes. The RCA jacks go to your headunit with aux-in or amplifier.
What? No aux in or amplifier? Look into a casette adapter.
What? No casette player? Look into an FM modulator. Wired ones produce better sound quality, but none of them produce very good sound quality.
Keep reading the FAQs. You have much to learn.