Troubleshooting Trailer Left Stop/Turn Lights Not Working with Curt # C56375 on a 2018 GMC Terrain

Updated 10/22/2018 | Published 10/20/2018

Products Featured in This Question

Curt T-Connector Vehicle Wiring Harness with 4-Pole Flat Trailer Connector

(73)

$23.18

16 Gauge Primary Wire - Black - Per Foot

(61)

$1.14

Question:

I have a 2018 GMC Terrain and have installed Curt 56375 trailer harness. The test light works and a buddy`s trailer with led lights works fine. When I hook it to my trailer with standard lights and the left brakes and turn signals will not work. Any suggestion? My trailer works fine with an other vehicle.

asked by: Paul

Expert Reply:

It sounds like you probably have a short on your left stop/turn circuit on your trailer end. It is very unlikely that the wiring on your 2018 GMC Terrain is messed up as it is a brand new vehicle so I would start by checking the connector on your trailer for corrosion or debris. If you have any fuses or relays on your Terrain dedicated to the trailer lights then they would be worth looking at as well.

If that looks good then check the wiring and grounds on that circuit. Look for places where the wiring is damaged or pinched and replace with 16 gauge wire # 16-1-1. Also make sure that your grounds are connected to a clean, bare-metal surface as even some paint can cause issues.

Attached is a helpful article about troubleshooting trailer wiring for you to reference as well. If neither of these solutions fix your problem than I recommend taking your towing setup to a trailer shop and having them take a look at your rig. Sometimes a fresh pair of eyes can spot something different.

expert reply by: Jon G