If I do not find a smoking gun in the trailer harness, I will certainly try
additional grounding. I have experimented some using gator clips on the
existing ground lug, but not in a very organized manner.
I did find one mystery ground wire running from the inside of the rear
driver side tail light cavity where the canopy light is wire passes through
all the way into the engine bay near the fuse block, following the path of
the e-brake cable under the truck. It is terminated to the canopy light
harness at the rear and floats free in the engine bay. I have tried
terminating this to ground, but there was no change in behavior.