I had the same problem mirror was pulling away and bouncing around when driving. I did see there was a gap between mirror and fiberglass.When I pulled mirror off the wood in wall was wet and rotted. So i busted up all the wood i could from mirror hole till it was somewhat solid. The let the area dry and helped it with a heat gun. Then I put wood shims in the hole area and then put expanding foam in let dry and trimmed it. Then put adhesive on mirror edge places anchors in holes and screwed it in.Then sealed outside of mirror. Seems to be holding fine and does not move.