Those torpedo-shaped fuses should be replaced every ten years or so. All of them in the fuse box. They are not expensive. But they do go bad over time.

So go get a new batch, and replace all with the same color/amperage, and throw away the old ones. They can burn with nothing being wrong, or not connect well anymore.

Greetings from my home town of Stuttgart, where I just visited the Mercedes Benz Museum two days ago (and bought a book on the W126)!

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Henry Bofinger
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)