You can get it replaced for $75 via the information on another thread on this board. I did it and they were wonderful to dealk with.
They will send you another entire mirror, and you will send them the defective one back. Do NOT under any circumstances try to take the mirror apart. If you do they will not honor the $75 replacement costs and you will be buying an $800 mirror from BMW.
Do another search and you'll find it. I'll try and see in a minute...
My rear view mirror also went bad but I just took a pattern of the glass to the local glass shop and they cut a duplicate mirror for a few dollars, which I then glass-glued on top of the old one. You can't tell the difference.
Of course, I have to move it manually at night, just like the old days!!
Whatever suits your taste!