I am dismantling a 1971 280Se (blown engine, parts car) and was removing the fuse block. After removal, I could see all the fuse block ground wires run to a central location under the left side of the dash.
There was a little rust here.
What a nighmarish place to check a circuit.

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1970 280S M130 engine- good runner
1971 250 M130 engine- #2 rod bearing, gone
1971 280SE (blown engine,parts car)
1977 German 280S W116-only 33 years old