I had the same problem for the longest time.. I had my windshield replaced from rocks kicking up and cracking it a while ago.. 2-3+ years ago.. and they didn't put the mirror back up.. well I tried every combo of super glue out there.. tried the window kits.. none worked.. I got a few combos of liquid nail and super glue to work for a while but all ended up falling after a few months.. well I left it down for a year cause was tired of it falling.. Now I got a crack in my radiator and bought some jb weld for a patch until my new one came in. Had some extra left over and mixed it together and it worked.. My mirror stays up.. been a month now so only time will tell.. it doesn't seem like its going anywhere.. I got the one with two separate containers black and red top.. mix them up WELL should be a dark gray almost black.. put a nice slab on.. don't be afraid to go to the edges.. Now then this won't be tacky so you need some REALLY sturdy tape.. I used packing tape.. and tape it up there.. make sure you make the tape tight against the window..
things you will need
1. Jb cold weld
2. rubbing alcohol/nail polish remover
3. q-tips
4. tape
5. towel/paper towels
6. razor blade (may need to clean old residue from window kit)
steps
1. Clean the window with rubbing alcohol/nail polish remover... razor if need be
2. put even portions of the jb weld on the back of package it came in... I tore a small piece off to mix with first.. about the size of a quarter of each.. (don't be scared a lot of it sticks)
3. After mixing it thoroughly use the small piece you tore off to spread it onto the mirror mount MAKE SURE YOU DO IT ON THE RIGHT SIDE.. would **** if you did wrong side..
4. slide it into the mirror (I had to mount it twice.. first time I did mirror mount alone and it did not stick..)
5. Pull out some tape.. about 2-3 feet long 4-6 pieces minimum..
6. start a piece of tape the will run horizontally across the base of the mirror.. the jb is NOT tacky so you will need to hold it till the tape does
7. Hold the mirror firmly in place as you run the tape across it.. smooth it down as much as possible.. Try to leave the very bottom open..
8. add two diagonal pieces off tape across the base of the mirror..
9. I started the tape on one side above the mirror and Did a U around it..
10. Reinforce your strips of tape with perpendicular pieces where needed..
11. Tape where necessary..
12. Now if you taped correctly the sides of the mirror base should have an inch or so of space you can stick a q-tip in a grab any excess weld off the sides.. after getting the majority you may need to dip them in rubbing alcohol to get the smudges.. DON'T let them dry or they may be permanent..
13. leave tape 16-24 hours.. don't turn up the radio either..