I replaced the light bulb – no difference
I replaced the fuse – no difference

How to troubleshoot from here?

TIA

Burn blub socket wiring

^Thanks for the comment.

Uh, I think that comment translates to something like “Check the voltage at the light bulb socket.” Is that correct?

Can someone tell me the next point behind the socket where I can check the wire for the high beam? That way I can check for continuity between the 2 points and see if there is a viable connection between the two.

The next point is the exterior lamp bulb failure unit which is the big relay in the relay box behind the fuse box. It goes in there and comes out of there and then goes to the fusebox.
More info here:
54 Chassis Electrical, Instruments

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1992 400E current
1988 300E sold
1973 280C totalled
1973 280SEL never got it running
1971 250C my first love but rusted out