Make sure the glass is totaly clean, use alcohol. The mirror backing must also be free of all residue and flat. I always use sandpaper on a flat surface to smooth the mating face, too fine a grade will only make it harder to attach. An abrasive surface 'keys' better and allows the bonding agent room to move, again use alcohol to cleanse. I use normal Loctite Superglue, just a couple of drops is required, capillary action does the rest.
Oh, and remove the mirror from the bracket, it's easy to handle and less weight hanging off it when 'going off', then replace mirror.
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