To fix it, you'll have to remove the mirror glass. The thin plastic frame of the mirror can be slowly and carefully pried off the rest of the housing, then the glass will come out and you'll have access.
Thanks, Jon, that got me on the right path.
A 1-1/4" wide steel putty knife was the ticket for scratch-free removal of the thin frame to pull the glass. Found that the housing was cracked where that stud penetrated that fastens the housing to the cast metal 'arm' covered by the bellows. Removal of the mirror glass was the
only way to see that surface.
Your memory is correct, a single 12mm nut holds the housing on the 'arm'. Removed the housing and I've injected cyanoacrylate (super glue) into the cracks from both sides, I'll let it cure overnight. We'll see how that holds-up and if not, I can see other ways to reinforce that plane of the housing now that I've been there.
Thanks again