Ape,
That's a common problem. Apologists won't blame Ford, but the simple fact is that the retraction of the seatbelts on the ST suck about 32 different flavors of arse. If not very careful and deliberate in getting out, it is likely that one will close the door on the seatbelt, and Murphy's law indicates that it will likely slam on that stop button, smashing it off.
It happened to me and so many others here.
I went to the dealer, they gave me a new one for FREE! Simple crimp on two piece deal (looks like a tack on one side, kind of, presses into a washer like deal on the other).
Stop by your dealer. Either way, if I remember right, it was a $4 part at my dealer.
TJR out!