If you want to try to extract them without cutting a hole in the A-pillar, click this, read its caption, & the NEXT several:
You won't remove the plate from the body, so you can't use a drill press, and the file won't go as deep. But you can essentially follow that process to get one bolt out. Then bolt it to the body so the plate doesn't slide around, and the others are easier. I'd start with the one closest to the center.