Rear Number plate mountings

Lost one the other day. They were the nylon type screws in there. Except when I tried to put a replacement in there was no way it was going to fit. So either these are the wrong ones, and a self tapper should be used or the threads are corroded.

Does anyone know what they should be? I am thinking the self tappers. I think that the garage I bought the car from have bodged these nylon screws in. As I cannot remove the other one too...

Any help appreciated!

Tom. '02 passat TDi sport

Self adhesive pads - end of.:D