dont know if its right but i used a good size flat head with a good(forgive the oxy-moron) gentle but firm hit/tap to knock it off.

Be forewarned, its not glass under the base (at least on my windshield) there seems to be a type of coating or something that was laid with the glass/window...

...i broke my windshield and had it replaced (insurance was good for something) and requested that they leave the mirror off. Which they did but it still had that base. And like i mentioned under the base it isnt clear glass so i no no good luck