One of my mirrors got blown apart a few weeks ago after hitting an extremely large grouse at highway speed with it. I had to replace the glass after putting it all back together on the side of the road.
If you adjust the glass so that it is roughly parallel to the mirror housing, then bend the mirror back toward the door, you should be able to whack the thick inboard end of the housing with a rubber mallet and it will pop off it's mount (outward). You can then carefully withdraw it past the glass. It's method of attachment and all else will become evident once it is off. I did it with the heel of my hand rather than a mallet. It worked but it hurt (a lot). Don't do that.