We have about three 06 crown vic police interceptors that the seat belt light stays on and will not go off. On one of the cars we have replaced the seat belts, the seat belt tensioner, and the wiring harness per ford which none of these has fixed the problem
Sometimes they have a weight sensor that goes bad and if it thinks someone is sitting in the passenger seat without putting on their seatbelt, also the switch in the buckle could be bad, it will throw that light. The best thing to do is to have the trouble codes pulled. Any Advance Auto or Auto Zone will do this for free.