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I have a 2008 GMC Envoy SLT. We tow a 20 foot boat with it. When we bought the boat, one of the trailer lights was out. We decided to update all the lights to LED’s and we rewired the entire trailer. The boat and trailer are 1997’s in excellent condition. The guy that rewired the trailer apparently nicked a wire by a marker light and he said there was an issue with the wiring harness connection.

We discovered the issue when we hooked up to the car and had no running lights but the turn signals and brakes worked. So he came out and rewired the trailer again. Stem to stern. Hooked it up to his car and everything worked fine. We hooked it up to our GMC and again had no running lights, but had brakes and turn signals.

So…..I am thinking a blown fuse in the GMC. I have a fuse diagram for the car and have found corresponding fuses marked as for trailer turning lights and trailer brakes. I do not find anything for the trailer running lights. Just fuse #32 marked for Trailer. Appears to be in the fuse block in the engine compartment. A rather large yellow fuse. Now, before I get in over my head here…..anyone have any insight as to which other fuses I might check? Never pulled a big yellow one before…. Any help would be much appreciated. Thanks.
 

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2004 gmc envoy_slt
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Ravalli, that wire is for charging a trailer battery and is left unconnected unless the user wants that feature.
 

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Yeah ... I just posted that in case he wants to be legal with brakes and a breakaway battery charging circuit.

Secondarily ... if that red wire is hooked up and fused, it's the ONLY way to get into a TB if the door lock mechanism is electrical (as mine is) and needs battery power to unlock the driver's door.

I can lock my door as that's just a mechanical motion ... but unlocking needs a charged battery to run the door lock motor.

If you get locked out, you'll wish you'd hooked up that red wire to charge the battery backwards through the 7-pin trailer electrical Jack under the rear bumper.

I'm on a mission to tell everyone about that.
 
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