electrical short

If this is a factory or factory "style" harness, and you still have the ammeter/ bulkhead connector, I'd start with that. Bulkhead connector, fuse link, ammeter circuit, don't forget the welded splice (in the black ammeter wire a few inches from the ammeter) and the ignition switch and connector

Wiggle wiggle WIGGLE. Strap a test lamp to the ignition power point, and wiggle everything you can get your hands on nearby, while watching the lamp