There are a few ways of getting the audio in to the car stereo. Easiest is some form of FM transmitter, either the FM Modulator or Belkin Tunecast or one of the others on the market. They are also the cheapest.

An option here a friend does with his WRX is he sits the ipod in a dock behind the gear stick and runs a cable into the FM input on the back of the head unit which has a splitter so he can put his normal FM input in as well. That is very good but like someone has said above, doesnt charge the iPod.

The best one that I have seen is:

http://www.mp3yourcar.com/

Only available in the US from what I have seen and does use the changer input on the back of the headunit.

HTH