I read some but not all of the responses.

When I cleaned my Window switches I found I had to scrap them to the bare metal with a small pocketknife. Contact cleaner or the Pencil Erasure that some people have claimed to use would not have worked in my case.

Any burned and black medal on top of the contacts is oxidized and not going to be a good conductor of electricity.

Another possible is that the Motor Brushes are getting short (not a short circuit) in size. There is some threads on taking the Motors apart.

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84 300D, 82 Volvo 244Gl Diesel