1: Turn off airbag with ignition key, (open passenger door insert key and turn to off on the side of the dash) as though you had a baby seat in the front. Turn on engine for a few mins then turn off, insert key into side of dash and turn the airbag back on.
If that doesnt work...
2: Try disconnecting the little white connector box under the passenger seat, should be clipped in place under the seat leave for 5 mins and then reconnect, light should go out after a couple of start ups...
3: Disconnect again and spray contacts with Electrical contact cleaner (from Halfrauds) and i mean really soak the contacts both ends leave for 5 mins or until it has evaporated and re-connect. Light should go out after a few start ups.
4: Seeing the age of your car and assuming the passenger seat has been used alot, either by people sitting on the seat or by sliding seat back and forth to let passengers in the back... its common for the sensor pad in the seat cushion has split (sensor pad tells the car to activate the passenger airbag when you have a passenger sitting down) (airbag will not go off in a crash if you dont have a passenger). This can be costly to replace as it means taking the seat apart replacing the sensor pad and re-covering. Hopefully its not this...
Also to bear in mind... not to keep bags on the passenger seat when driving as the right weight of even a heavy briefcase can effect the sensor pad in the cushion and cause unessecery wear.
Hope this helps