Have you probed the plug directly? I recently had the same problem. Drove me nuts trying to figure it out. Turned out somehow the connections weren't making contact. I used a pair of needle nosed pliers and squeezed the connectors tighter and cleaned them really good with a gun cleaning brass brush, and put some dielectric grease on them and it worked fine.

 

I could be wrong but I think your truck has fuses for trailer lights. Id look in the fuse box first to see.

 

Fuse box under hood.they are there,lists location under lid .

 

Yep, bad fuse under the hood. My 2017 did that for the running lights on the trailer.
Replaced the fuse and all is good now.

 

On my 2017 2500, I use to plung in the trailer wire connection to the truck and then start my truck. Worked great. Then last year I did the same thing and none of the left lights worked on the trailer. They all worked on the truck and I had power to the outlet. I checked the fuse and it was good. I turned off the truck and unplugged the trailer. I then started my truck and plugged the trailer back into the truck. On the dash it said trailer is connected and all of the lights worked. Once the truck recognize the trailer was hooked up all was good. Even when I would stop to fill up and turn the truck off and when I started truck back up, all the lights worked. So, try having your truck running and then plug in the trailer. It doesn't make sense to me, but that is how I got mine to work and everytime that I have hooked up the trailer since then with the truck running, all of the lights worked.

 

Use a meter and see if you have any voltage at the plug. If no voltage than you probably have a bad fuse or broken wire.
If you have low voltage than you have a short or a “draw” somewhere.
My truck did the exact same thing. Truck had all lights working but every trailer I used did not have left tail light working.
My voltage on that trailer plug was only 2volts.
Turns out I had a tail light bulb on the truck that worked BUT the plastic base had been hot enough to start melting and was using so much voltage there wasn’t enough left to make the trailer light work. I replaced the truck bulb and now everything works great on the trailers.

 

It’s a fuse. If you can’t find it, check them all in the fuse panel under the hood.