Don't know if it actually works but I heard that removing a mirror from the mounting and popping the head/Mirror into quite hot water will sometimes enable you to get the class out of the groove in the mirror head to enable access to the fixing inside, tighten the fixing up and reheat the plastic mirror head back up and clip the mirror glass back in.
NOW I HAVE NEVER TRIED THIS SO I DON'T KNOW IF IT WORKS, BE IT ON YOUR OWN HEAD IF YOU TRY.
If you try and it works let us know, if you try and you cut you hand on the broken mirror class or end up with a bit in your eye I don't want to know.
As I said it is hearsay as I have never heard from anyone who actually says it works in the real world.
Might be worth trying if the mirror is completely shagged, but wear eye protectors and gloves.
tommy lights the fuse and retires a safe distance.
Just been out to the shed and took a look at my mirrors and to be honest I'm not convinced its possible, but somehow they got the glass in there during manifacture, and the only access is from the front. Dunno