Are you using Flat 4 wire connection between truck & trailer? I bought a flat 4 wire Key chain tester. If I ever have light issue first thing I do is plug tester into the truck, turn on parking/running lights & hazards. If all 3 light work on tester, then the issue is the trailer (usually bad ground). They probaly make tester for the other style trailer connections but not sure, or a simple volt meter will do to test the truck.
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