Need a suggestion for a project I'm working on

I have a project that I have been working on and I'm at a tough spot and need some suggestions.

What it is is a car stereo with a 5" resistive touch screen. I have grafted in my own USB touch screen controller and will use a toggle switch to go between the onboard and offboard controllers. All that is shaping up great but I need to figure out how to get the USB to the offboard controller.

The faceplate houses both controllers and has a terminal block that communicates with the head unit itself. I was considering hijacking some of the pins but I think I can only get 3 from the USB port on the faceplate (which is worthless and can only see 2gb drives) but I need four wires for USB, right? This would let me cut the pins in the chassis and run a USB socket to the back of the radio and greatly simplify things.

I'm guessing they get away with using 5v, D- and D+ with a chassis ground - would I be able to do the same as long as I tie the Gnd from the offboard controller to the faceplate's ground?

The other idea is just have a wire hanging out of the faceplate that gets plugged into a USB cable and then the CarPC itself but that is pretty ugly.

So far this seems to be turning out very nicely. The only bad thing is the display is composite and 16:9 aspect so the CarPC output will be stretched and I don't think any video cards can compensate for that. I will have a DPDT slide switch on the bottom edge of the faceplate to toggle the X- and Y- between on and offboard controllers. I removed the pins and sockets from the offboard controller to slip it inside the back of the faceplate and have everything connected with magnet wire due to size constraints.

I'll see if I can get some pictures up later - nothing 'in progress' as I wasn't too concerned with that and the finished product is pretty self-explanatory.