I had replaced the mirror couple times because of my paint job...
1st: you need to put the ball thing behind the mirror on top of the hole from the housing..(move the mirror left and right to see you put the ball in the right place)
2nd: apply pressure from the other side... Don't put too much force.. you might break the mirror
3rd: You should hear a click sound when the ball thing go into the hole...
4rd: repeat the first 3 steps til you are done...

Good Luck