Replaced 20 amp fuse, closest to fender, 3rd up from fire wall.

 

do you have stop lights and turn signals on the trailer working ? Power for all except the tail light ? Check for a good ground wire. Pull the plug and check voltage on the brown wire to ground.

 

I'm having the same problem. Turn signals work, running lights do not. I traced the problem all the way back to the connector under the bed of the truck and found the brown wire at the input side to the connector has no voltage when the headlights are on. Asked the dealer for a wiring diagram of this and he said he didn't have one. In looking through what he did have, we discovered that there is a relay for JUST the running lights and a fuse for the running lights and the reverse light. I circled both in white so you can see them. I haven't tried anything with them yet, as I just got back and was getting on here to see if anyone had a wiring diagram. I can't believe the dealer didn't have one.