Turn Signal also feeding all the clearance lights on that side.

Try another pigtail.

 

Sounds like someone hooked up the wires in the 7 pin plug wrong. I'd start there.

 

Sounds like somebody mixed something up somewhere. But being that is a pretty new trailer, idk. I know if you cross the wires up that feed the lights, the 12v signal can activate the bright side of the light. Maybe unplug your lights on that side one at time first, see if you have a weird one causing a circuit malfunction. Then you can test each pigtail supply side for 12v at the turn signal prongs and see if your getting a constant on the wrong feed

 

That would be my suggestion, too. Or at least grab a test light and see that the right turn signal is ONLY powering the right turn signal pin, and that the marker lights are ONLY powering the marker light pin. That will tell you if the problem is with the truck or the trailer. Start at the pigtail, and work your way toward the problem.

 

Disconnect the trailer cord, check with voltmeter just tractor first. If everything looks good there, then it's in the trailer wiring. If you're sure it's the truck, there's usually 2 junction boxes. 1st is in driver-side jockey box, and connects all tail-light/backup/headache rack and trailer wiring. 2nd is on the exterior frame rail, near the rear drive axle. Check these junctions as the exterior one is prone to corrosion. If all is good there, then you'll have to work your way back to dash switches and signal switch. Wiring is a real PITA to diagnose, but you'll eventually find the problem.

 

Power cord could be shorting inside. Try a nee cord or one that you know to be good.

 

Sounds like a grounding issue to me. 9 out of ten times if something electrical was working and starts flaking out, it's a ground issue.

 

Bad ground seems to cause many problems at once . Like said check pigtail if problem replace or unplug first from Truck and check prongs on Truck . I had that problem had to change 7 wire from inside cab to rear receptacle where pigtail plugs in.First thing is figuring out if trailer or truck. If trailer the plug for pigtail first stop

 

If the pins are adjacent, it could just be a loose strand that isn't properly contained in its terminal bridging the gap to the other. Once you figure out which end (truck vs trailer) is the problem, start opening things up, using the test light, and work towards the problem...eventually, you'll find it.