[rpmerf]

I was going to comment that for the 3rd brake light, but I wasn't sure about the wiring for 1985.
A dedicated wire from the brake pedal switch would likely be most appropriate. Look at how they do it on newer trucks.
A standard 4 pin trailer plug should have ground, running lights, left, and right.

Not sure on how to test the module. Still seems like a grounding issue though. I think it acts normally without the module because the wiper switch does not need a ground to operate. Just a guess though.