Ford's and my trailer lights
 

I have two fords, 05' and 08' that won't work the trailer lights on my trailer. Yet, the 06' dodge and 98' s10 do. Why? The trailer is open with do-it-yourself wiring.

None of the lights work? What about those trucks on other trailers?

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.