[PBJ2500CTD]

Your symptoms are typical of a bad ground. We are just having a difficult time identifying its location. What I find odd is the symptoms are occurring with your truck and your trailer; with your truck and another trailer; with another truck and your trailer. That last combination would indicate a bad ground in both your truck AND trailer, which can be a possibility. The only combination not given was how the other truck and other trailer acted.

You said you removed some tape and did not see evidence of corrosion at the harness addition. How were the wires connected? Were they soldered, or stripped and wrapped together?

I agree with BG Smith's recommendation for a starting point. Disconnect and clean the harness connector to plug...maybe even change the 7 pin plug out. They aren't that expensive.

Have you changed any bulbs in either the truck or trailer lately? Even if you have not changed bulbs I would verify that the the correct bulb is installed in each lamp assembly and that they are clean and properly seated. Check the elements in the bulbs, too. Make sure they are positioned on their respective posts. Another item that has come to mind since the symptoms happen to both left and right is to try disconnecting the brake controller. Now if you could only get access the other "TDR" (time domain reflectometer) we could find your issue in short order.