As the fuse instantly popped, I’d usually agree with the wiring fault theory.

This is a pretty simple harness. There’s the + and - trigger wires, they connect to the factory compressor clutch harness. All that does is supply signal voltage to close the relay. Then there’s a + wire that has an in line fuse that leads to the relay. This wire is tied into the glow plug fuse, but could go to the battery instead. Finally there’s a relay switched output wire that goes to the fan. All of these wires have less than 0.2 ohms resistance, probably less (DVOM sucks).

Relay has almost zero resistance when energized

Rollguy supplies a ground wire for the fan, so the fan is directly grounded to the inner fender

Maybe there’s crazy high resistance between one of the + wires and ground?