proceedure to remove rear view mirrror

i have to replcae my rearview mirror on 1999 buick regal gs. i got the replacement part, but i cannot figure out how to remove the old mirror. there does not seem to be a set screw visible on the old mirror like the one on my new mirror? is there a plastic cap covering it? there seems to be a small protrusion at the lower front of the fastener but can't seem to get into it. does anyone have a suggestion"
JT