Yep it's a pain once it falls off as none of the recommended adhesives, even the strong bonds with pieces of gauze do the job.
The surface area of the securing pad is just too small for the weight of the huge mirror so what to do is cut a piece of aluminium abiout 1 1/2" square (35mm),rough up one side and stick the mirror securing pad to that without the mirror attached. Leave it alone for a day to really harden.
In the meantime free up the mirror adjustment by putting some engine valve grinding paste on the ball socket for mirror mevement and work this in vigorously as this is often so stiff that it puts too great a strain on the securing pad. Once you've worked it loose (could take a while) then wash out the paste and put a little lubricating grease on the balljoint.
Next day, thorughly clean off the glass ready for fixing the mirror pad alone. Make suer you ermove any pervfious attempts at fixing bond. Now watch it, as the pad has to go quite low on the window to allow the mirror to slide down from above. Check out the positioning very carefully as there's no moving afterwards.
Then use the strongest mirror bond you can find to fix the aluminium plate to the window, make sure it's square (looks just fine) and leave it overnight.
Then fix your mirror!
