I recently got a used flatbed trailer and nothing electrical works. I am redoing all the wiring. The lights are no problem, but I am not 100% sure on the brakes. It's got 3 MH axles all with brakes and each one has 2 wires. It looks like the PO joined them all into 2 wires and ran them to the plug. I haven't seen anything grounded to the trailer. Is this correct or should I ground the brakes to the trailer as well as through the plug?

 

The brakes really don't care if they are grounded to the frame. They must be grounded through the trailer plug. Both won't hurt. Lights should be grounded to the frame for simpler wiring.