Troubleshooting Curt T-Connector On 2018 GMC Terrain Not Having Left Turn Signal/Brake Light

Updated 09/29/2021 | Published 09/27/2021

Question:

I have installed the wire harness to the factory plug. everything works except the left signal and brake light. The light works from the plug with a test light. When you hook the trailer up you loose power to the plug. The trailer wiring is good. I eliminated the trailer and used a single bulb it blinks once and you loose the power to the plug. after 30-45 seconds power comes back to plug. Thank you

asked by: Gary I

Expert Reply:

That is a pretty weird issue!

I would start by checking that the connector for the Curt T-Connector Wiring Harness # C56375 you installed in your 2018 GMC Terrain is fully seated into the factory connector.

If that is the case then go ahead and remove the connector and check both the Curt and Factory connectors for any debris or corrosion that may have worked its way in there causing your issue.

Next go ahead and check that the two relays in the fuse box of the rear cargo area are completely installed and are not causing a weird issue. I would go ahead and try to move the relays from one spot to the other to see if the problem moves or not. If it does then the issue is the relay is bad.

I included a video of the # C56375 for you to take a look at.

expert reply by: Samuel C