One way to tell if it is the Motor is to sit a Car Battery near the Door and make some Jumper Wires to Hot Wire the Motor directly.
Don't have experience on your Year and Model but on the W123s the Motor is not grounded to the Chassis as part of the Electrical Circuit like your Starter Motor is. (It could be your Mechanic does not realize this; I also though My Motor was dead till I tried it.)

If you have both Motor Wires in hand connect the Jumper Wires one to one terminal and wire and the other to; the other termnal and wire and the window will go one direction.

Swapping the Wires on the Battery Terminl cause the window to go the opposite direction.
If the Window work like that you have an issue somewhere else.

One of the other issues is that sometimes the Brushes inside of the Motor are worn. People have gone inside and replaced the Brushes.

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84 300D, 82 Volvo 244Gl Diesel