Yes this happened to me. It is a real problem with these cars. Have two Contours, and had the same problem on both.
Either you snap the screw head, or the metal thread insert starts to turn.
The entire license plate "holder" or assembly is actually plastic, and can be removed from inside the trunk lid.
I removed mine and drilled out the bolt and rethreaded it. It is a metric M5 thread I think.
It might be worth your while to see how much a new plastic assembly costs.
