I managed to break the casting on my manual mirror. It had seized and I tried to force it up......with the result you've had!!!!
However it does mean I've got a spare to play with, so I just have.
I hope this is all relevant for an electric mirror???
The mirror glass is held on to its plastic base by an adhesive backing pad almost the same size as the mirror itself. I've just managed to remove it so its possible. You will need three wide chisels and a thin craft knife. Pull the longest side of the mirror level with the casing, you should just be able to see the plastic backing. Start by running the knife along the join a few times. Then put a chisel into the gap and GENTLY rock it sideways a few times (in line with the mirror not levering the mirror off), do the same at the ends. Rocking is more controllable than just pushing down. Any undue flexing of the mirror will of course break it. Continue to do this, cutting the backing wherever possible and use the chisels to gently force a gap for the knife. Took me about ten minutes. Eye protection and gloves needed [img]smile.gif[/img]
Another method could be to use a solvent on the adhesive pad to dissolve it, but not sure if this would damage casing if overused.
Yet another and perhaps more satisfying way would be to just smash the mirror [img]biggrin.gif[/img]
Once the mirror is removed you can access the four screws that hold the casting to the plastic.
Not sure about the other end of the casting as mine is just bolted on, being manual.
Hope this is relevant to yours (and not too long winded/obvious!!!)