had the same problem in a car i was borrowing while the sprint was off the road

mate got in, knocked the mirror off with his face

all i did to fix it was get good old superglue, squirt it on the bit of the mirror that came off the window, whack it on the glass, get something to hold it there for a while, takes a good hour, good as new

that car is still in the family and has stuck for 2 years now