I think I mentioned this in another thread, but:
HiFiSoundConnection.com sells the Zune Core Pack for your car. You can get it for around $40.00. It comes with the core unit, which hooks up to the AUX port (RCA l/r/vid out) through a 3.5mm a/v jack. It also has a feature that lets you hardwire it or use the lighter adapter. You can charge AND play through the Zune plug with out having to use the headphone jack.
(it's the square black thing in your pic.)

You'll also need to get the plug for the back of the head unit, 3.5mm end to plug in to this.