Think my car stereo must be like Charlie's. I have a power plug which goes into the cigarette lighter socket and then a connection which goes from the earphone socket on the mp3 and into a hole on the front of the car stereo. You can press an 'aux' button and everything seems to work. Marvellous, this new fangled stuff!
Before I got this, I tried all sorts of wiring around the car, which meant a quick exit from the passenger seat was a nightmare!