There is a factory fuseholder that is recommended for the '87 300D (W124). The same thing would probably work on your W123 and may be recommended. I wasn't aware of it, not having had this problem with my 123. You can get a pigtail fuse holder from an auto parts store and use whatever kind of fuse it takes. Make sure the wires are stout as the fan on high speed draws about 27 Amps.

__________________

"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