The other day, it seemed like there was something funny with my driver's side window going up or down, like it was straining a bit, but then it went about its business, and I blew it off as though it were just having a bad day. A few times since, the window worked fine.

This evening, I tried to put the window down, absolutely nothing (at least it died in the 'up' position!!). I checked the switch by changing it with another good switch, still won't go down. I can hear the window getting the power, but no window movement.

I'm assuming some electric motor in the door, is toast? If so, is there a way to fix that, or just replace?

jeff 1991 300d 169k

Does your 1991 300D have just one driver's side window?

W124

Those above comments make it sound like it is an issue with the Regulator.
However, you won't know untill you pull the Door Panel off and have a look.

With the Door Panel off You may also find a way to hot wire the Door Motor to see if it works.

On My year and model the Power Window Motors are not grounded like a Starter Motor is as part of the electrical circuit. You have to put the electical power direcly into the 2 wires.
Swapping the direction of the Electrical Current Flow changes the direction of the Motor.

__________________
84 300D, 82 Volvo 244Gl Diesel

If the earlier ones have the same plastic spool, cable arrangement as the later 124, and 210, its is likely the reg. Although they typically die in a not quite closed position.

If it is "the regulator", are we talking about the motor, or the gearing? I'm assuming you replace the motor rather than repair it, and if the gearing just lube it up or something?