Back License Plate Mount

[Ron Soderberg]

Got new license plates for the 1990 Audi 100. Well, I mounted the front
plate with no problem. The back plate is a different story. Minnesota
winters and road salt have done a major rust job on the screw
attachments. So instead of the screw simply coming out, what Audi used
for a nut is spinnning like a bad carnaval ride. I can see I can't get
to where I need to be from the back. Any ideas for getting the damm
screws off short of breaking the plastic surround. I would think this a
pretty common problem. Thanks everyone.