Update:

I went ahead and removed the regulator assembly from the door. I bench tested the motor and assembly using a marine battery with no luck. I removed the motor from the black round gear housing and found that the motor was binding on the inside shaft bearing. I worked it loose by pulling up using pliers, and was able to get the shaft to rotate, although not what I considered freely. I bench tested the motor again and it worked fine in both directions. I reattached the motor to the gear housing and bench tested it again. It rotated to the (window) closed position, but then became stuck again. I guess that the force applied to the motor shaft when the arm meets the bump stop is pushing the shaft into the inside damaged bearing causing the motor to stick. Unfortunately, I don't think I can disassemble the motor to inspect the bottom bearing, or clean and lubricate it.

What is the most cost effective way of reliably repairing the assembly? I have read that the motor is not available separately, and I must buy the entire regulator assembly. Does this includes the cast metal support and slide mechanism, or just the motor and gear housing? Where do you recommend purchasing one (retail or dismantler)?