Here are the hot tips.

The windshield has to be SUPER CLEAN.

The windshield can not be hot (i.e. - do this at night).

It has to be held tight against the windshield for a reasonable amount of time. If you do not want to do this by hand, use a broom handle or something like that between the mount pad and the seat back.

Oh yes, the glue can not support the entire weight of the mirror till it has cured, so remove the mounting pad from the mirror before you do any of this.