Just something for people to think about:
I have a 2005 Outback Sport, a rock hit my windshield and I had to have it replaced. The glass place got the replacement windshield from PPG. While I was driving home from the place I tried to adjust the rear view mirror and it cam off in my hand. I went back to the place and we tried to figure out what the problem is. The bracket that the mirror slides into is already attached to the windshield when they get them from PPG so it is not the original bracket. The installer got my old bracket and we compared it to the new one, visually there seems to be no difference, however when you slide the mirror onto the old bracket you can hear a definite click as it locks into place, on the new one you never get that click.
They ordering another windshield today and are planning to test the mirror on this windshield before they do the replacement again. It may just be a bad bracket was on the first windshield.
Anyway, if you have to get your windshield replaced really check out the mirror!