Troubleshooting Running Lights and Brake Lights on Trailer Connected to 2004 Toyota Tacoma  

Question:

Expert Reply:

If you are using an aftermarket 4-pole wiring harness like # 118379 or # C55379 in your 2004 Toyota Tacoma you can troubleshoot the lighting issue in several ways.

First, check that the truck's own lights all work correctly. The harnesses like those noted above plug in to the truck tail light assemblies to get the lighting signals. If the truck's own lights do not function there will be no signals for the harness. If you unplug the harness and test the truck's lights and find that they do NOT work then check your fuses in the power distribution box.

If the truck lights DO work then re-install the harness, making sure to fully seat the connectors at the vehicle light plugs. Make sure there is no corrosion, no bent contacts, etc.

Also make sure that the main white ground wire for the harness is firmly attached to a clean bare metal surface to make a good ground. Weak ground connections can cause all sorts of problems with wiring. You can sand off that grounding point to expose clean bare metal or you can create a new grounding point. Either way I suggest protecting that grounding location with dielectric grease like # 11755.

If you have a circuit tester like # PTW2993 you can have a helper activate the vehicle light functions on the truck while you apply the tester, as shown in the linked video.

You can also test the trailer directly by unplugging it from the vehicle and applying +12V power directly to each circuit, one at a time, to see if the trailer functions. If yes, then the issue is in the vehicle wiring; if no, then check the wiring on the trailer for a loose or corroded ground connection or for damage to the wiring.

expert reply by: Adam R

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