Might try finding a heavy duty flasher for your truck to see if that helps. Sometimes the cars are not equipped with heavy enough turn signal flashers to run the car lights and a trailers lights.
If your wires are wired up correctly then it's the flasher in need of being beefed up. Most auto stores will have heavey duty turn signal flashers for automobiles
Check the ground also to make sure that your lights are properly grounded with the White Wire.
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