That's a tough one to answer. In theory the frame is a ground (negative) for the entire 12 volt
system and you should be able to ground locally to the wheels. However, that gives you four more points of possible problems. If I were to do it, I would use a common ground wire and make a super good single ground connection to one point on the frame, or even to the negative lead which comes from the TV cord (or both).
You want brakes to be reliable, always, and they are critical safety issue items. I guess I learned a lesson when at 65 mph, I came very, very close to rear ending a compact car and pushing him under a tractor/trailer. I found 3 of the 4 brakes on my AS were not working. Yikes! Two were wiring problems, one was a mechanical issue. Never again.