Fuse: Check the fuse responsible for the left side trailer blinker/brake light. If the fuse is blown, replace it with a new one of the same amperage.
Ground: Ensure that the ground wire for the left side trailer blinker/brake light is properly connected. Clean any corrosion or rust from the ground wire and ensure it is making good contact with the metal frame of the vehicle.
Wiring: Inspect the wiring for the left side trailer blinker/brake light for any damage or corrosion. Repair or replace any damaged wires.
Trailer Connector: Check the trailer connector to ensure it is clean and free of corrosion. Make sure the pins in the connector are not damaged or bent.
Trailer Light Assembly: Inspect the trailer light assembly for any damage or corrosion. Replace the light assembly if necessary.
Truck Light Connector: Make sure the trailer connector on the truck side works by testing other trailers
Brake switch.
