I'd say it depends on what you are wiring. Obviously the positive (red) goes to the upfitter switch. If you're wiring a light or something the ground (black) can go to any "grounded bare metal". If you're wiring electronics like a radio, GPS system, or something like that, I'd run a lead from the negative terminal of the battery, which is what I did. Just my 2 cents....but bare metal would probably work in either case. I just a have a preference for the battery connection if it's an electronic toy.