Fair enough, @GeorgeSanJose ; if the appendectomy analogy reflects the difficulty of this repair, I’m not above getting it done at a garage. To answer your questions above, no, I don’t have a voltmeter; when I said that I tested the wires at the switch, I was referring to a test light. When I first got a quote on this repair, the mechanic had a quick look in the door: he didn’t report any electrical problems, but simply said that the motor/regulator unit should be replaced since he believed the motor to be defunct. When I asked if we could just replace the motor, he replied that his source didn’t sell them separately.
So what would be especially helpful is if there was some way that I could confirm, within reasonable probabilities, whether it’s just the motor that’s shot, or the regulator too. While I’d be paying for an hour of shop time either way, the cost difference if I supply the part (especially if it’s just the motor and not the whole unit) is significant.
Alternately, I can buy used-but-apparently-well-functioning regulator/motor units on eBay; again they’re much more affordable than the shop quote for the new factory part. I wonder if this would be worth the cost savings, or if it’s too risky to buy sight-unseen?
It would certainly be nice to watch repairs being done; unfortunately, in my experience a lot of shops aren’t fond of this, and can make it difficult logistically alone (by giving vague and long time estimates).