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

Yeah you're right. Not a good idea. BTW glow plug resistance is more like 0.7 to 0.9 ohms.

Is it possible you have the wrong fuse (like a 240D fuse) in there? Are the fuses marked as to how many amps they are? Take a look!

IIRC 616, 617, 601, 602 and 603 engines all use an 80-amp glow relay fuse until MB switched to self-resetting breakers. The strip fuse is commonly available in 30-amp (aux AC blower fuse) and 50-amp ratings.

Sixto
83 300SD

The fuses I've used are definitely 80A, it was stamped on them. That was what I had in the 240D as well (I hope it was right!) and I did already pilfer that one, alas. I will have to remember it's not there whenever I finally go to start that car again, ha. I thought I bought more of them at some point, but can't find them.
I'll see if HF has that ammeter you mention, sixto. If I have time before it closes.

It's getting hot on supply end, just a little warm on load end. The fuse blows right at the bottom of it, next to the screw. I see what you're saying about plugging them in one at a time now ... will try it once I get more fuses.

Some auto parts stores sell a MAX I - FUSE holder. You can wire that in place of the strip fuse and pop in an 80 amp maxi fuse.

This is bad advice for a fuseholder. It will be fine in THIS case, because the strip fuse is a solid tinned socket and screw terminal.

BUT the pop in fuse holder is copper plated STEEL, so sanding or wire brushing is a VERY bad idea!

The advice given is for the strip fuse. You are the one who brought up the pop in fuse holder, which is NOT plated steel.

So the same 80A fuse is used for engines with 4 to 6 glow plugs?

The size of the fuse is not that important as long as it is not too small (will burn out). For testing till you find the problem I'd suggest using plumber's solder as a fuse (1/8" dia) and hammer it flat. It'll be a lot cheaper than blowing real fuses.

Honda uses a 70 amp strip fuse that's stocked on the shelf of most auto parts stores. it would be ok to use for testing out...

while it would indeed be fine to just clean it with a wire brush, it DAMAGES the components when you do this.

like I said, for the Glow plug fuse, a wire brush would be ok, but not the best.

metal polish will reverse oxidation, and rinse clean with contact cleaner.

it's best to do this on parts you don't want to replace when they wear down to nothing. cleaning fuse contacts with abrasive like pencil erasers, or wire brushes will damage the fuse holder surface. it will WORK, but it's not correct, or the best practice.

for battery terminals, and cable ends, a wire brush is fine! they are parts you replace often.

fuse holders are NOT. wearing down the surfaces will alter the fuse characteristics, and eventually lead to the need for replacement.

I've been told that the MB push in fuse holders are steel... if they're not, what are they? bronze? either way, they are plated, and abrasion alters the surface and leads to failure.

Don't believe everything you've been told! Find out for yourself! Does the pop in fuse holder contacts stick to a magnet? Scrape a corner of it with a knife, do you see any plating come off?

65Amp Honda Acura 65A Flat Metal Strip Fuse Made in Japan Ships Fast | eBay

At $7 a pop, I'd use plumber's solder unless you have money to burn.

Edit:
I have a glow plug fuse I made from plumber's solder for my VW when I didn't have a real fuse when it blew. I tested it by shorting the glow plugs and the fuse I made blew so it worked just fine (didn't smoke any wires). That was 3 years ago and the fuse I made is still there, working fine.