ive used crazy glue in the past, it seems to work as good as the special mirror glue they sell, just razor blade the windshield clean and the metal part you are gluing on, and then clean with rubbing alcohol, put glue on the metal part, and press where you want it to live, hold it for a few minutes, let it set up, over night at least. don't try to put the mirror right back on, it will fall off.
and watch out you don't put it on upside down, been there done that, one time lol