So no one has done this yet...
I've started on mine and it looks like you will need to break the RF part open to get inside it. There are no outside screws visible and the damn thing is like glued together. Making it real hard to pry apart without damaging it
I'm planning on wiring in a stereo jack into the RF box so that i can have it wire directly to my stereo in. If i get it open i will try to take some pictures.
edit: Okay didn't take long to do. I took pictures along the way. Bent the outer case and then hammered it back in place when i was done
Plugged it into my car and bam, worked like a charm. No more car charger and stereo plug and no more RF

since i cut the audio wires. Maybe now that it works i will go back and completly remove the RF box.
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Sorry for double posting back to back. But here it is.
Zune Car Charger
Simple pictorial of my mod today.