We've seen a couple of instances where the door motor sounded like it was trying to work, but simply didn't. IIRC there'* an article around about removing the motor and lubricating it. This has been found to help.
I'd doubt it is the regulator based on our lack of seeing bad regulators on the 92-99 era. (Seems to be one thing they got right) That appears to be isolated to the 2000+ crowd.
Off to find that lubing thread.
Edit: Here we go. I'd try the greasing and lubrication first, then consider a motor if that doesn't work.