The proper way to turn the air bag light off on the dash of your 1991 Ford Crown Victoria is to repair the fault in the air bag system that is causing the light to come on. When the air bag light is on fault codes are stored in the air bag control module. The first step in diagnosing your air bag system would be to retrieve any stored fault codes. The fault codes should then be diagnose and repaired as necessary.
I had this happen to my 2001 ford crown vic P71. I was messing around with some wires under the drivers seat when I disconnected the two white wires leading from the seat belt. (I made a plan to disable the No seat belt light and tone warning by cutting one or both of the wires or by simply connecting them) My plan worked but had the side effect of making the air bag light blink on and off. I have no intention of causing malfunction to my airbags for the seat belt thing and I just re-connected them.
My first answer is to check the fuses. You can get the owners manual online from ford. My second answer would be to check the wiring from the seat belt on the driver or passenger side.