How to Reattach a Rearview Mirror ?

One bad pothole later, the SUV disappears suddenly — not because the driver has given up on passing you so he can get to Starbucks 8 seconds sooner, but because your inside rearview mirror has dropped off the windshield glass and is resting peacefully on the carpet in the passenger foot well.
Stick to It

Most vehicles rely on a simple glue joint to hold the rearview mirror onto the windshield. The adhesive can fail due to the ravages of heat, humidity and time — not to mention the added stress of a half-pound of fuzzy dice or kewpie doll ornament swinging back and forth for years.

Fortunately, it's relatively simple to reglue the metal mounting button to the glass. Unfortunately, it's not so simple to reglue it properly, so that it remains on the glass for more than a few days. Just a dab of 5-minute epoxy won't cut it.
Your first step is to head for an auto parts store or local big-box retailer with an auto parts section. Pick up a rearview mirror installation kit. Don't buy one that's hanging on a hook from your local odd-lot-merchandise emporium or any other place that isn't likely to have a fresh kit on hand. These kits have a limited shelf life, especially if not stored properly. Similarly, don't buy one and keep it stashed in your hot glovebox — it won't last.
Many cars nowadays integrate electrochromic dimming functions, flux gate compasses and even courtesy lights with their interior rearview mirrors. If some functionality of the mirror has failed, or the glass is cracked, you probably can simply replace it without regluing the button to the glass.
Have a Nice Day
You'll need to park your car in a place where the temperature is somewhere between 50 and 75 degrees. If it's winter, find a heated garage to work inside. During the summer, pick a nice day, with moderate temperature and humidity. Park in the shade, because if the glass is too hot, the adhesive will set with less strength. Rainy days will leave moisture on the glass.
First, you'll need to remove the metal button from the mirror assembly. Very few (usually older) vehicles have mirrors that glue directly to the glass, but most will have a button you glue on first and then attach the mirror to. The Audi mirror we replaced here can be forced to part company with its button by removing a small piece of trim, disconnecting the electrics, and then inserting a screwdriver blade into the latch. Twist gently, and the mirror will pop off into your hand. Other vehicles may require loosening a small Allen-head setscrew.
Very important: Find some way to label the button "this side up." The last thing you need is to glue on the button upside down. Mark it with a felt pen or a scratch at the very top or bottom. Don't mark the side that faces the glass, because the next thing we're going to do with the button is clean that surface down to bare metal. Also, you'll be looking at that surface from outside your car for a few years. Do you really want to see a smiley face there?
A Circle Marks the Spot
There are probably some remnants of the glue on the windshield. Before you clean anything up, use a felt pen, grease pencil or a piece of masking tape to mark the position of the button. For reasons that are about to become obvious, mark the outside of the glass. If nothing else, there is probably a constellation of your fingerprints on the inside of the glass from adjusting the mirror in the past to guide you. If you are very short or very tall, you might want to use this opportunity to adjust the position of the mirror an inch or two vertically. Don't forget that there might be wires to connect if you have any electrical components in the mirror. If you have trouble hanging pictures level in your living room, you might want to add a plumb line to the windshield with a spirit level so you can keep the button square. We usually just eyeball it.
Cleanup on Aisle Three
Remove every last vestige of old adhesive from the button. The kit we bought had a piece of sandpaper in it for this. If there is any adhesive left on the glass, scrape it off with a single-edge razor blade. Follow up with some solvent, such as lacquer thinner or rubbing alcohol, to remove your fingerprints from both the glass and the button. (Now you know why we marked the outside of the glass.)
The kit we used had a special cleaner/primer pad saturated with a solution that did double duty. A solvent cleans the glass and also acts as a catalyst for setting the adhesive later. Scrub the glass and the button without touching the surface with your fingers. You don't want to leave any oily fingerprints behind — they will prevent full adhesion.
Ready? Car and glass at a moderate temperature? Got the button right side up? If so, open the small vial of adhesive with a razor blade and squeeze a single, generous drop out onto the face of the button. Squint, aim precisely, and press the button onto the glass. You get only one chance, so we really do mean aim precisely. Hold the button in place for 1 minute, using moderate pressure. The directions in the kit say for 10 seconds, but we're professional skeptics. Similarly, the directions say you can install the mirror after only 30 minutes have elapsed, but we prefer to wait overnight to be sure the adhesive has cured properly.
Hang 'Em High
Now it's a simple matter of reinstalling the mirror. If you have a setscrew-style attachment, it's easy. Just hold the mirror in place with one hand while you run the setscrew in with the other. Remember, it's a setscrew — don't strip the threads by overtightening it. Use just enough torque on the wrench to keep the mirror from rattling.
Our Audi was a little trickier; your vehicle may be, too. We needed to rotate the mirror around the octagonal piece of the button to pop it on. After that, it was a simple matter to reconnect the electrical wiring harness and reinstall the small trimpiece.
Unbuttoning the Button
Upgrading to a better mirror because you have flux gate compass envy? Or you wanted to recycle a mirror from another car, say, that junker rusting slowly away out by the fence line? Then you'll need to remove the button from the glass. You might be able to shave it off with that razor blade. If it won't come off readily, here's a trick: Take a crescent wrench and "unscrew" the button from the glass. It'll pop off with a surprisingly gentle amount of force.

Mark the position of the button on the outside of the windshield with a felt pen or grease pencil.

Pop the button loose. Some mirrors come off by removing a setscrew, but this one had spring-loaded fingers that had to be gently persuaded with a screwdriver.