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Troubleshooting No Brake Lights from 2016 GMC Terrain Wiring Harness

Question:

I have running lights and turn signals but no brake lights and its a 2106 GMC terrain i bought the wiring harness for this vehicle that is plug in style

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Expert Reply:

The first thing to check is that your harness connectors are fully seated together. Also it can happen that one of the metal contacts in the connectors (either the vehicle or harness side) could be bent and this could prevent the brake light signal from reaching your trailer.

Also check the main ground wire for the Terrain's wiring harness; this white ground wire must make metal-to-metal contact with the vehicle. You can try cleaning/sanding the grounding point, moving the ground wire to an existing ground location for other wiring, or create a new grounding point. A weak ground can cause all sorts of weird behavior from a wiring harness, including one function that does not work.

You can also check the Terrain's power distribution/fuse box to see if there may be a blown fuse for the brake light circuit.

Since I do not know which specific harness you are using I want to mention that the plug-in type, like # 118264 and # C56210, are for vehicles that have the factory tow package. Vehicles that do not have the tow package require a different type of harness, a powered/hardwired type like Tow Ready # 118494.

If none of the above steps corrects the issue then you may want to test the connector on the Terrain using a circuit tester like # 40376. Have a helper step on the brake while you apply the tester one at a time to the green and yellow wires. You should detect a continuous signal on both wires when the brakes are engaged. A video is linked to show how to do this.

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