So from what I can tell, there are two feeds to the GI in the back box. Upper and lower (not including flashers). The bottom GI works, the top, not so much. Traced all the wires to the power supply, but cannot seem to find the root cause why the top half of the backbox GI won't light up. I checked the fuses, and swapped them around and the working lights/fuses still worked, while the non-working upper backbox GI continues not to work.

Thanks wayout! Newbie mistake. Assumed the molex connectors were good...continuity test showed the lower backbox GI feeds (brown and brown stripe) had continuity, but the green and green stripe did not. Found loose connection at the 3p8 connector, re-seated it, continuity galore...and top backbox GI now works as it should!

Thanks again!