Rear view mirrors on the inside of older cars are attached directly to the inside of the windshield.  They can get knocked off or the weather, over time can affect the glue so they just fall off.  You can get glues for this at the auto parts store.  They are epoxies(two part) and are pretty good.  You can also use Mister Glue for this – metal to glass.  If the surfaces are cleaned properly and you get a good tight fit, the bond should last a year or two.  On some of the rear view mirrors there is a little piece of metal that the main part of the mirror slides into.  Ideally, you want to remove this and glue it by itself.  Put a bit of glue on the metal piece(you only want the glue on one side) and press it tightly to the glass.  Put something, a piece of tape or whatever on the outside so you know exactly where to place it.  Any glue that leaks out and runs down, leave it and scrape off with an Exacto knife once dried up a bit.  Once fully cured, carefully attach the main part of the mirror.  One VERY IMPORTANT thing, though – make sure that you attach that little piece that the mirror slides onto the right way up!!

I had an outside mirror on an older car where the glass got smashed out on the drivers side.  I got a piece of mirror from the auto parts store, cut it to size and glued it in with Mister Glue and it lasted for 5-6 years – I sold the car – it is likely still there.  It worked perfectly  The beauty is that Mister Glue is water proof so the weather will not affect it.

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