Got pics? IIRC the rear plate bracket is plastic frame with metal threaded inserts that go into the trunk. Usually the issue is the license plate screws themselves rust so you end up stripping them when trying to remove. Post pics and let's see what you're dealing with but seems like you'll need new inserts. Also while you're at it, go replace the screws with stainless ones. If you're not running a license plate frame, a M5x8 is perfect. If you're running a frame obviously you'll need a longer screw. ECS sells these starting at 12mm
here.