to narrow your search all four wheels either work or don't work correct?
If that is correct then make sure you have good connections on both sides of the trailer plug and truck receptacle. work back into the trailer from the breakout location where the 7 wires separate to run the lights brakes etc. to the first connection at the left side of the front axle. Eye balling the connections won't tell you a thing. When I suffered the loss of all 4 wheel brakes it was a bad crimp connection where the 7 wire cord spliced onto the trailer wiring.
Good luck