Help needed sorting an electrical issue

[E.D.75GMC]

Alright guys, I need help. I just put a rust free box on my truck the other day after having spent the last few weeks grinding and painting the frame and bottom of said box. Just got around to installing the tail lights and harness and noticed a small problem. The drivers side tail light does not work when you hit the brakes or flash when you put on the signal light or hazards. (Note that the drivers side front signal works fine) Now, you're probably thinking "well that's easy!", but here's the thing; since I first noticed this issue I have replaced not only the bulb, but the flasher unit, as well as swapped out the whole harness for the extra one I had in the garage and none of them had an effect on it. Do any of you guys here have an idea? I'm pretty stumped.