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The most common problem with trailer lights is the ground, make sure it is grounded properly, even if the connection is tight, take it off and clean it.
You can also use a test light on the connector on the truck to make sure there is power there by turning on a blinker or the hazards and probing for power. Then you know for sure it is the trailer.
Check that out and let me know what you find.
Back from my trip. Left turn signal works and marker lights. I was told by local trailer supply that 2011 GMC trucks are wired different than my 2005 Chevrolet truck.
Truck connection is good with both blinkers working.
Can I set up 12 volt input to trailer connection to determine blinker and brake contacts?