I think it was mentioned, the wires where they come out of the motor, there is a small plastic junction box. Within an inch or so of where the wires come out of the small junction box (it's maybe 1 inch by 1/2", black plastic box on the motor), the wires fatigue (fray) most of the way through under the insulation (probably from years of vibration). What you end up with is a few threads of wire to conduct the electricity. It makes the motor run really slow if at all. There are other spots, but this one is common. Sometimes the switches corrode and don't pass enough juice.
You have to take the motor and assembly out to fix the wire, unless your an elf with tiny hands. Cut off the funny spade connectors they have on there and about three inches of wire, crimp (or solder) on a pair of standard spade connectors and exit the opposite side of the junction box.
I also grease up the cable and cable guide and spray out the motor with air and contact spray. Sometimes grease and dirt gets into the brushes.
I've only replaced on motor since 1987, I've managed to fix all the others.
I also solder the electric seat wire and the power window wire together under the drivers side carpet and run an extra ground (solder a short piece of wire in and screw it to the chassis, I used the dash brace just over the drivers side kick panel) from the window harness in the same bundle of wires.
This almost doubled window speed, but does bypass the ignition switch in the circuit, the windows work without the key being on and the window lock doesn't work anymore.