Here's an adapter if you want good quality sound. I'm thinking of actually getting this instead of an FM transmitter >>
http://www.crutchfield.com/S-wiRAJO3...o&i=469GM12AX2
The page is specific to Chevy Cavalier but you can change the model/make
Here are some good and cheap FM transmitters:
http://www.roadmasterusa.com/fmmod_vr3.php
The top of the line one has an SD slot so you can play mp3's or WMA's just by inserting your card.