I don't know about fixing it, but if it's anything like the M3's where people have videos of taking the entire side mirror off to do chrome delete, you should be able to take the apart to replace. Looks like there's plenty of examples of MX mirror removals for reference.
I watched my chrome delete installer take it apart. Not overly complicated, but you have to be comfortable with removing trim pieces as that are held in place mostly by plastic clips.