I forgot to ask -- your 8 volt measurement was made with someone's foot on the brake and you back at the stop lamp? That means that somewhere there's a wire that has either corroded, as winmutt says, or has mostly broken so that there are only one or two strands, not enough to carry the current needed to light the stop lamp.

The only alternative is an electronic failure in the bulb failure module, N7. You would have to get a spare (junkyards should have them or someone on the forum) and do a swap to see if it helps. The part number is 126 542 01 32 and they are common to all 124s and some 126s (S-class cars like the 300SD and 300SE, etc.). You don't want to know what the dealer would charge for a new one.

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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