I have a 2006 F-150 with trailer light issues. Let me start by saying:

1) I have redone the ground connection at the plug by cutting the wire and reconnecting to a new place on the frame. I wire brushed the frame to bare metal to ensure a good ground.
2) My adapter goes from a round 7 pin to a flat 4.
3) When I check my voltages with a voltmeter at 7 pin and flat 4 they all check OK.

Now is where my issue is. When I connect a set of lights (not on my trailer} to the flat 4 I get the following conditions:

• Running lights work
• When I turn either turn signal on, both lights flash
• No brake lights. When I hit the brakes all lights go out.
• No 4 way flashers.

These same conditions exist when I connect my trailer so I think the problem is with the truck. I am at a loss and looking for suggestions.

 

12V on the left or right wire lights both sides.... Your left and right wires have a common point that shouldn't exist. Easy enough to perform a continuity check between the two pins on the trailer and also on the truck.

Brake causes all lights to go out... sounds like it's putting 12v on the trailer ground. Easy enough to check the trailer body for 12V, and also the ground pin at the truck.

No 4-way... now it gets interesting, as hazards is nothing more than putting 12V on both the left and right wires at the same time. Should light up both lights as using either turn signal does the same. If one of the two wires was connected to ground, you would never light that bulb.

Check that your truck is good... 12v alternating on the left and right independently, and also during hazard. Ground is ground, brake is brake, etc. Eliminate the truck, then do continuity checks on the trailer. Start at the plug, make sure no wires see the other wires.

 

<Edited because reading fails me sometimes...>