90 degree pliers? To be honest in the interests of speed and simplicity, I concur with the viewpoint of fragmenting whatever rusty junk is holding the plate on by using a largish diameter drill bit, pull off the plate, then use either mole grips or 90 degree pliers to get some purchase on the exposed stump and twist it out (or snap it off, in which case drill it flush and screw new screws into the plastic behind the numberplate - or as David says above, use a few strips of good quality 3M VHB foam tape to hold the plate in place.