2006 F150 FX4. Trailer plug had been changed from the round 7 pin to the little square 4 pin when I purchased the truck. I changed it back and noticed that a white wire had been cut coming out of the main wiring harness at the back. Everything still works as it should on boat trailer. I'm going to use a brake controller in a couple of days to tow an enclosed heavier trailer. Would this white wire that's clipped be part of the wiring from controller plug under the dash? Haven't picked up trailer yet so I haven't tried to see of brakes on it function properly.

 

The white wire is usually the frame ground wire and runs from the truck plug to frame at back of truck. You can find white wire screwed to frame on drivers side.Trailer lights may or may not work without it because 12v to lights may return thru hitch ball to truck frame. Not good way to do it.

I would meter that white wire (to see if its shorted to frame) if so run it to ground pin on truck 7-pin.

 

Just in case you don't have it, here is the trailer wiring diagrams and color codes for 4 thru 7 pin connectors.
https://www.etrailer.com/faq-wiring.aspx