"Rearview Mirror Glue", sold at any Auto Parts store for under $5
(I just had to do this during my last WDW visit

)
It's a 5 minute job, M's, you can do it
...The key is to mark the spot with a crayon on the OUTSIDE of the windshield (trace outline where old glue is) , then thoroughly clean off all the old glue from the windshield (use alcohol and a razor blade...or something alcohol based...i didnt have any, so I used a dab of mouthwash on a napkin

). You'll need an allen wrench to remove the 'button' from the back of the mirror (I ran over to the
AAA car care center near the MK and had a mechanic do this for me

) There's a primer in the kit, for the back of the button...takes 2 minutes to cure, then only ONE drop of the glue. Press the button to the windshield and hold for a minute... then, let it stay button ONLY overnight before re-hanging the mirror.