Electrical issue.
I went to hook up the black tank flush the other day and noticed the back-up light on. OK, obviously something charged that circuit, a worn wire or something. I plugged into the truck and the truck ba...
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I would open up the wiring junction box on your pin box where the trailer cord terminates. There will be a wire from the 7 pin cord with a wire for the backup lights connected to a trailer wire, likely with a wire nut. Disconnect that wire from the cord. That will at least stop the trailer from backfeeding and turning on the truck backup lights. You might even find the wire problem there. Could be the backup light wire has somehow come in contact with the 12 volt wire in that junction box.
7 pin wiring
7 pin wiring
