I had one of my Front Windows stop working and was due to the 2 Screws that held the Motor Wires to the Terminal Block being loose. You can see them if you are careful to pry the lowest front part of the Door Panel away from the Door.

There is a couple of thread on removing the Counsel and the Switches and taking the Swich apart and cleaning the burned contacts.

don't lose the 2 Ball Bearings or 2 Springs inside of the Switch.

When it happened to me I used a small Pocket Knife to scrap the burned part off of the contacts.
Also sometimes the Switch is just full of gunk. However, I found Brake Cleaner makes the Plastic sticky so I would not use that.

If not the 2 items above next is a tie between the Motor Brushes being worn out or a Wire where the Door Hinges being broken.

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84 300D, 82 Volvo 244Gl Diesel