Troubleshooting Intermittent Power on 2017 Mercedes GLS 450 7-Way
Updated 12/15/2025 | Published 11/22/2024
Question:
I have M Benz GLS 450 SUV with factory trailer wiring. I also added the adapter for trhe LED trailer lights and things worked fine for a while. Now I am not seeing any power to the Original 7 pin plug that the adapter plugs into. I check all fuses under back seat that are identified as trailer related. I have no clue about the other two fuse boxes- one under hood and other at the right end of the dash by the glove compartment. It is a bit intermittent, as one day last week I had Brake/turn SIgnals, but no tail lights. Today nothing and nothing at the plug, with my 12v tester. Any ideas?
asked by: Patrick B
Expert Reply:
Hi Patrick; since it is intermittent it sounds like something is loose or there is a short. I would start by checking your ground wire to make sure it is secure on a clean, bare-metal surface. If the ground is good I would try looking online for some fuse box diagrams so you can check all of the fuses for the trailer connector functions.
You could also just open the other fuse boxes and look under the lid for a diagram. It would be a good idea to make sure they're properly seated in there since that can also cause intermittent power loss. If that all checks out then you probably need to trace the wiring back as you test while looking for a short. Depending on how the wire is run this may be a job better suited for your local dealer.
Product Page this Question was Asked From
7-Way to 4-Way/5-Way Trailer Adapter for Vehicles with Lamp-Out Sensors Towing Trailers with LEDs
- Custom Fit Vehicle Wiring
- Wiring Adapters
- Universal Fit
- 7 Blade to 4 Flat
- 7 Blade to 5 Flat
- Plug Only
- CURT
more information >
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Robin H.
12/15/2025
Hey Patrick, did checking the ground and fuse boxes on your GLS 450 help get power back to the 7-way plug? Your update could make it easier for others dealing with the same intermittent trailer light issue.