Factory plug is getting 12 volts for left and right brake and signal lights, but will not light a bulb. Bulb was tested on the running circuit and works. both filaments
asked by: Dale
If the problem is on the trailer side, you may need to check under the hood to determine if there is a fuse or relay missing or not working. You can use the vehicle owners manual to find the towing related fuses and relays.
The load from the trailer may be sufficient enough power draw to shut down the tow package wiring on the truck if those fuses/relays are not in place.
You can also check to make sure that the trailer tail lights are properly grounded to a clean and corrosion free bare metal surface and that the pins for the turn and brake lights on the trailer are in the correct location for the pins on the truck side connector.
Finally, any dirt or corrosion in the trailer connectors can cause problems like this so it would be a good idea to clean them out. Corrosion can also form at the backs of the connectors so check those as well.