I ran one pair of wires per drum and led them all forward to a terminal strip. That makes any future problem diagnosis easier to accomplish.
You can ground the (-) phase of the brake wiring to any clean frame point. The size of the steel in the frame will preclude any large voltage drop from resistance in the circuit. If you do choose to ground each (-) brake wire, individually to the frame, you will have six (6) 12V frame ground points to maintain, four from the brakes, the battery's frame ground, and the ground from the towing connector umbilical.
