[Formerly Steve Wilkinson]

Well, if you were trying to scare it, that might work...

Put some penetrant on it overnight. Then tap it--the mirror stem--at the bottom, lightly. Lots of taps rather than one enthusiastic whack, and as I said do it with a non-metallic mallet.

The groove in the mirror pad is an interference fit; it tapers slightly and grips the mirror stem harder the farther in you put it.

I have two mirror pads on my windshield--one for highway mirror location, one for track--and I move the mirror frequently from one to the other. It takes a little effort, due to the interference fit, even though mine is by no means corroded in place. Don't be hard on it. There's a lot of stress entrapped in windshield glass, and it's not hard to make it let go.