Ford's and my trailer lights
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The number one reason I run into for trailer lights not working is a bad ground. (Bad fuses are number 2.) Yes, the white wire is supposed to be the ground, but they also ground back through the trailer frame to the ball on the receiver hitch. Sometimes just bouncing up and down (moving the trailer on the ball) or driving around a few tight corners wakes them up by removing the corrosion that's insulating the ground. The other way you can test for this is to run a temporary jumper ground from the trailer frame to the truck frame. You;d be surprised how often this is the problem.