Check to see if the trailer plug is factory T-Plug style with factory style connectors or if it spliced into the wiring.
If it is a factory plug, a couple of things could be wrong. Without knowing make and model of the vehicle, it would be had to say if it was a tail-light assembly or some other factory connection.
If the plug is spliced into the truck, the reverse light may be tied in erroneously. Easy way to check. Get a voltmeter or test light, Turn off all lights, place vehicle on level ground set brake, chock wheels,turn ignition to the run position without startingengine and place vehicle in reverse.
Remember. Engine Off, vehicle on level ground set brake and chock wheels!
Move to rear of vehicle and check plug-in with test light or volt gage using the 7-pin wire chart.
You should not have any voltage on any terminal except 12-volt pin #4 and the center pin if it is wired for auxiliary power.
If you do not find voltage on any other pin, then the problem is in the trailer wiring. If you find voltage check all splices and connections.
Check the Brake Control at the fuse panel or where it is spliced in to the wiring. Use a wiring diagram for your vehicle for color determination, this can be found in the owner’s manual or a repair book. More than likely, the reverse switch is wired into the electric brake wire somewhere.
If you cant find any issues with the truck check the wiring on the trailer at the plug connector, If it is recently converted from a flat or other pin style it could have a stray wire from the #4 pin or center, making contact in the plug connector. If this check out then check the brake wiring connections. One of the hubs could be improperly wired so when you go to back up one of the brakes drag.
However, the obvious question may be, do you have surge brakes?
