[mjw357]

Originally Posted by vipzach

Is the wiring correct? There should be a wire that connects to your reverse lights that will disengage the brakes when in reverse.

You are thinking of surge brakes. If his foot is not on the brake pedal, there should be zero braking.

What kind of controller do you have? How is it mounted?

Before you pull the wheel, do a little test. Try backing up again like you did with everything connected. If it locks, unplug your harness and leave it unplugged. Try backing or driving forwards. If it is still locked, you have a mechanical issue with the brakes. If it goes away, it may be an electrical issue.

Originally Posted by walleeme

Sounds like the emergency breakaway switch on the trailer gave a bad signal.
Look at that first.

Yes. That switch is normally open until the cable pulls the pin.