Ugh, sorry to hear how fast a tedious new wiring loom can get too old. At least you solved which side I'll put my main fuse on the battery -
I just went through this on a '96 Olds Cutlass. Someone prior had put in remote starter and used the dash light wire for the main current path. Discovered on return from Barber's when stopped for no tail lights, which are common power with dash lights though two separate fuses to confuse. i put in 30 amp fuse instead of 20 as gave that away to a biker. Got 100 yd away when smoke then flame leaped out the glove box fuse box!!!!! Did last 150 mile on hazard flashers.
Had to take dash off to the firewall to track down 8 EIGHT spots where insulation melted to short with bumps and blow fuse. Every single place was where the loom was bent, clamped to a mount or passed through a grommet That may be clue where to open your loom to look in. We sliced each fault area and so far so good.
I'd try it with CO2 type fire exhtingisher at hand and power up everything and watch for smoke or feel for heat, maybe after dark to reveal some sparks? Power up and tug and jerk wires too, if nothing found may have lucked out.
Power type extinguisher require lots of clean up and can inhale in the excitement of the moment.
I don't know if the reverse current damaged your Zener, maybe voltage check and hand feel would reveal.
