I don't know how similar to the 300D your 300SD's electrical system is . . . but in the '85 300D (W123 body) there's an over-voltage protection relay (OVP) that powers all of the gauges and some other electrical things. It has its own 10 Amp fuse. I don't know where it is in your car, possibly in with other relays near the fuse box? In the 300D it is hidden up behind the passenger kick panel.

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"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