The airbag light signals a problem with the airbag. A common cause of this problem, is when the instruments have been unplugged and the ignition was not turned of. The airbag controller then recognizes that there is no connection to the instrument cluster and stores the fault. Another cause for the later production years is the collector ring being damaged. This collector ring connects the airbag in the steering wheel.
The best way to solve this problem is to connect your car to a diagnosis system. If you have any electronics knowledge, you can even build such an diagnosis interface by yourself. http://www.e34.de/tips_tricks/…eicher/fehlerspeicher.htm (German)
With the help of the diagnosis you can read the stored fault, and delete it, in the case of an false fault.
Sometimes also the airbag controller after 01/94 has the problem that it thinks that there is a passenger airbag, but there is none installed. The reason for this can come by an unplugged battery for a longer time. In this case also deleting the fault will solve the problem.