1994 STS Seville

Mirror fell off while driving.... bought the rearmirror installation from autozone,
I know I should remove the botton( a piece of metal in recutangular shape) from the mirror, stick it to the windshield for 24 hours before I put the mirror on.

However I can not remove the botton from the mirror, it is snapped into place and there no screw, I tried to use the pliers to press down the plastic flange holding it in but it won't come out. Any idea?

Also the adhesive seems not so powerful and I don't think it can even hold the botton alone,.

Thanks!

called dealer, they ask $25 to fix it..