My mp3 player has a USB plug built in, and my new car stereo has a socket that says 'Aux' where you can link it to your mp3 player. It's like a small headphone socket. What sort of cable is needed for this and where can I get it?
Not a case of choosing - your car input will definitely be a simple audio input. You'll need the 3.5mm-3.5mm cable above, and probably an in-car USB/iPod charger that you can plug into the cigarette lighter socket. Like this