heat. now this being a gun try a hair dryer on high for a bit to warm the screw and its mating hole. (i normally use a torch, but thats on the @*#&$@*&$ CAR, NOT A FINE PIECE OF EQUIPMENT).
They may have used loc-tte (not sure why but is possible) and heat will often break that free too.
then again, i could be wrong (thats the lawyer line)