HWats wrong with my friends truck?

My friend's 98 F150 was bought as a salvage truck. IT was all repaired, minus a drivers airbag. So because there was no airbag the airbag light was always on. Well it burned out eventually. That was about 8 months ago. So as soon the the light burned out the truck started to beep. 5 sets of 5 beeps,. It starts whenever he starts the truck, it also happens when he rapidly accelerates or decelerates like coming of the highway or goes over some rough road. What can he do to fix this? whats the problem? and if need bhow do u replace the light?