Chasing No left turn Signal on Trailer With Ram Truck

I had a similar problem, but with a Ford. 10 years ago I added airbags. This year the trailer lights stopped working. Fuse OK, relay OK, now what?
I traced the wiring inch by inch and found that the tube to add air to the airbags was rubbing against the wiring harness. 10 years later, to wore through a wire. No damage to the airbag air tube, however.
Maybe no help to you, but you may have to trace the wire to see where it is broken.