Any chance of pics? It's been a looong time since I've had the trim pad of a late 70's car off. I can see the motor has a gear, but what I can't recall is how that interfaces with the window itself. Has it got one of those long arc shaped gears? So that the gear on the motor moves that arc shaped gear which has followers on it that moves the window mechanism?
Myself, I would disconnect the window from the regulator. I used to take duct tape and tape the window in place by taping over the top of the window frame. If there is no window frame, you may have to get a friend to hold up the glass.
With the glass disconnected from the window regulator, run it off the switch and see if it goes up and down smoothly. What you need to determine is if the window regulator is what's binding or if the window is binding the regulator. If that makes any sense.
If you can, disconnect mechanically the motor from the regulator. See if the motor runs smoothly.
You have to try each individual part of the entire mechanism that raises and lowers the window to see where the real problem lies. The motor looked to be pretty inexpensive, but if that's not the problem it's just money wasted. The motor makes a lot of revolutions to either raise or lower the window. I think it's highly unlikely that the motor only turns just so far in either direction. It's got to be in the regulator or the guides for the window itself. Be sure to look down in the bottom of the door for any pieces that may have fallen off. A roller or guide could have come loose and be causing the whole problem.
I'll help more if I can, but from here it's kinda tough.
Edit: Doing a little research and it looks like it might be a tough job to disconnect the glass. They used some special fasteners with two slightly elongated holes in them. You need a spanner wrench or the special tool for the job. I've improvised in the past with needle nosed pliers.
The regulator assembly is probably a variation on a scissors lift. How the motor interfaces with that I truly don't recall.
A mention was made of a stripped gear. How about a foreign body lodged between one of the teeth not allowing it to turn? That would jam it both going forward and reverse.
I think you're going to find a part, or a guide or a bolt down in the bottom of the door. Find where it came from and your problem will be solved. I doubt the regulator mechanism "just failed", something has come loose or fallen off. Basically there is a guide at both the leading edge and trailing edge of the window. A "U" shaped bracket attaches to the window using two or possibly three of those nuts with the slightly elongated holes. The regulator mechanism has two rollers that run in that "U" bracket. The rollers get closer together to raise the window and they get farther apart to lower the window. As I said I just can't recall how the motor connects to the regulator.
Be cognizant of what is actually holding that window up when you start disconnecting things. That sucker will drop in a big hurry. Big doors and big, heavy glass.
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