I donno, its hard to diagnose from here
However, there is really only 1 thing that can go wrong within that mechanism, and it suspiciously made from really cheap, thin, brittle, plastic. And its integral to the whole drive and probably the MOST stressed part within the whole thing.
I mean, Im not sayin' GM purposely stuck this plastic spur gear in there so they could sell more $1200 mirror assemblies or anything, but they might have !
Hahaha.
I'd crack it open, have a look. And dont be too shocked if it is that plastic gear in pieces.