Harbor Freight used to sell an automotive ammeter gauge for under $5 which I used to find grounds that popped fuses instantly; eg failed AC clutch bearing and 124 rear window wiring at the hinge. Check craigslist and wrecking yards for something you can use as a coarse indicator of whether it's safe to use your precision voltmeter.

Is the fuse terminal getting hot on the supply end or load end? It's possible you have an oddball glow plug grounding when hot. Thus the suggestion to hook them up to the glow relay one at a time to find the culprit. Connect glow plug 1 then go through a cycle. Add glow plug 2 then go through a cycle... until the fuse blows.

Sixto
83 300SD