Hello, hate to say it but the motors inside the mirror are not serviceable. The glass is designed to be replaced, but only after it has been broken. Procedure below from GM for replacing a broken glass, I don't know if it will help your situation. The only thing you might be able to do is lubricate the plastic jack screws that run the glass in different directions. You may be able to access part of them from behind the glass if you use a spray like WD40 from around and behind the glass with it tipped all of the way in one direction and then the other. With a spray nozzle on the can of WD.
Apply slight pressure on the jack screws at the helical gear openings.
This action pulls the up/down jack screw into the motor pack.
This action pulls the left/right jack screw into the motor pack.
Important: Proper alignment of the trunnion prevents damage to the motor pack trunnion receptor area.
Manipulate the mirror back in order to align the mirror back trunnion into the motor pack receptor area.
Important: Do not strike the glass. Uniformly apply increasing pressure.
Firmly press the glass toward the motor pack in order to achieve proper insertion.
The insertion is complete when the mirror back trunnion nests in the motor pack trunnion area.
Observe for the following conditions that indicate a successful installation:
Auto Tech
Plus 40 years GM, over 30 certified with Chevrolet, ASE certified Master Tech. (Expired, Retired)