DC current clamps are rare but available. I have an ancient HP-428B (ancient = uses vacuum tubes) that works great but its range tops out at 10 Amps. Modern solid-state versions are also available but at much greater cost than AC clamp-on ammeters.

Inductive DC ammeters (hold meter next to wire) are perhaps the best compromise. They have relatively poor resolution but are easy to use and not expensive. Available in various ranges, I don't remember whether the smallest would work for one glow plug.

Here's a link to a 30-0-30 Amp meter that would work.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970