The adhesive used to bond a rear view mirror to the inside of your vehicle's wind shield has the strength to endure the heat of the sun. When the adhesive fails and you have to replace it, you must first clear the glass of all the old adhesive to ensure a proper bond when the new adhesive is applied. The old glue will not remove with standard window cleaners or most solvents, but you can strip the adhesive with little effort to leave a proper bonding surface for your mirror.
Hold the handle of a window scraper blade slightly off the surface of the window. Set the scraper blade against the glass one inch from the adhesive.
Move the scraper rapidly as you slowly move forward across the area with adhesive. Use longer strokes to remove the last remaining adhesive from the glass. Stop scraping when the scraper glides across the glass without catching on material.
Pour isopropyl alcohol on a clean rag.
Clean the area that contained adhesive with the alcohol dampened rag.
Wipe the windshield with a lint-free cloth to remove remaining streaks and fingerprints from the glass.
Wear rubber gloves to avoid drying out your skin when using isopropyl alcohol. Leave vehicle windows down to dissipate alcohol fumes.