Help with wiring on a trailer

[shilala]

I re-read your initial post, Bob.
You should only have two wires at the left side. One is the tail/marker and one is the flashed lead for the turn signal.
So the brake/running light (tail/marker) goes back on one lead. Should be brown, IIRC.
The flasher should be yellow on one side and green on the other, I think.
The ground should be white.

What you describe acts like a grounding problem. The low amp lights will work, but when the max amps are applied the ground is insufficient to get the job done.
If one side is good, I'd think it's the ground at the fixture.

In my experience the best place to lose a ground, aside from at a grounding point, is where guys splice wires with a crimp connector. You can pull on it and it seems connected, but the corroded wires inside aren't grounded well enough.
It'll make a guy lose his mind cause everything looks good.
I've been bit enough times that I take time to solder wires and heat shrink everything nowadays. I even bought a heat gun.

Good luck, brother.
Let us know what it was, will ya?