Welcome Mark.

This forum is for gasoline-engine cars, there is a diesel forum for diesels.

However, the W140 is really uncommon in a diesel, so ...

Mercedes made some degrading wiring harnesses, perhaps that's why "Things degraded from there ..." My Some cars had the harnesses repaired, some are replaced, others replaced as needed, yours might fall in the 2nd category and possibly there's a new issue somewhere between your start switch and the horn relay, ...

Do you know if your car has had the harness replaced/repaired? You can look for crumbling insulation under the hood. I do not know for sure that the '92 has a suspect harness, somewhere on this and the diesel forum there is more information, search wire harness to find out.

I have a '93 SD (soon to be scrapped), harness looks good in mine, but I do not know the history of my car before 2009.

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Gone to the dark side

- Jeff