It sometimes happens that years of exposure to the sun's rays can weaken the adhesive holding the rearview mirror to the windshield and the mirror will come off as a result. Driving without a rearview mirror is not only dangerous, it may also be illegal and lead to a hefty fine. There's no need to panic. Your local auto-parts store and many discount and home-improvement stores carry rearview mirror adhesive, which you can use to re-affix the mirror right at home.
Remove the old mounting button from the fallen rearview mirror with a screwdriver or Allen wrench.
Place a mark on the outside of the windshield with a grease pen at the spot where the old glue lies. This will give you an indication where the rearview mirror should be attached.
Scrape off the old mounting adhesive from the inside of the window with a razor blade. Clean off any residue left behind with a soft cloth that has been soaked in rubbing alcohol.
Squeeze the plastic activator pen, which is found in the rearview mirror installation kit, to move the contents of the vial to the pen's felt tip. Apply the activator to the inside of the windshield at the mounting location. Allow the activator to dry for two minutes.
Remove the tip of the adhesive bottle with a pair of scissors. Apply one drop of the glue to the centre of the smooth side of the new mounting button, found in the installation kit. You can also use the old mounting button, if you prefer. Scrape off any adhesive residue first, as you did for the window in Step 2.
Press the mounting button onto the activator directly after applying the adhesive. Hold the mounting button in place for one minute to allow the glue to set.
Allow the glue to dry for 15 minutes and attach the rearview mirror to the mounting button. Secure the mirror to the button and tighten the setscrew with a screwdriver or Allen wrench.
Allow the glue to dry for longer than 15 minutes, if possible even overnight. This will ensure that the mounting button is held onto the windshield even more securely.