Apple Component AV cable

I'm mounting a 3G in my car to hear music & watch video. It all works except for the charging part. I cut off the USB end and found 3 wires + the outside shield of mesh. Any idea of what is ground and what is 12 volt so I can hook it to my car and get it charged while watching videos?
The cable is made for home use (110 volt) but I want to adapt it to 12 volt because no one (including Apple) makes what I need for my car.