Glow plug relay is new, mechanic had an extra, and I swapped it. Problem not solved.

Actually, with some investigating, I think I know the problem.

The ground wire (after the fourth cylinder) looks old and frayed. I moved it around a bit, and there was a bit of sparking, so the connection looks bad.

Where can I find a proper ground wire, because I tried splitting it, and I failed.

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1980 240D - 275,000 KM