Troubleshooting Issues With Trailer Lights Working/Not Working With Truck Lights On/Off
Updated 01/19/2026 | Published 08/04/2024
Question:
I have 2008 toyota tundra. When the truck lights r off the turn signals and brake lights work, but no running lights? when I turn lights on the truck the running lights on trailer work but turn signals and brake lights do not? Please help
asked by: Scot
Expert Reply:
Hi Scot; this sounds like a classic ground issue. Typically when you have a bizarre situation like yours it is a bad ground. Make sure the ground wire on your trailer and vehicle isn't damaged. You also need to make sure both are secure on a clean, bare-metal surface.
If the ground is good on both go ahead and test the connection on your 2008 Tundra with a circuit tester like # PTW2993. If you have power on the turn signals/brakes and running lights with the truck lights on and off then it is an issue with the trailer. If you don't, you know it is an issue with the vehicle wiring.
If it is on the vehicle side try testing right behind the connector and work your way back. If it is part of the factory harness plugged into the rear connector you can replace it with the # HM11141835 you were looking at. If not, you will have to continue to trace it back until you find the problem.
If it is on the trailer side you will still want to work your way back while testing until you find the problem. If you need to replace the trailer side connector you can use the # 002402.
Robin H.
1/19/2026
Hey Scot, how did things turn out after checking the grounds and testing your Tundra’s wiring with the circuit tester? Knowing whether the issue was on the truck or trailer side could really help others with the same problem.