In reply to trek:
If you have an ipod then your best option is to change your car stereo to one that has an aux jack on the front and yu can then simply plug in your ipod! I got a Kenwood cd tuner with ipod jack for 45 dabs...
Bonus is that all of your music libary is available and not one cd left in the car. Also, if you have kids, you can put there fave films, cartoon etc on the ipod and they can watch with sound via the car speakers. A cheaper option but not quite as good <but you can still use in other peoples cars> is to but an audio sender. these plug into the ipod and sends an fm signal to your car stereo. these are only about a tenner but not as good as option 1.