Does anybody have information on why air bag light stays on all the time?

FWIW this was posted in the wrong forum subsection - this is the e28 forum. But as an e39 owner who has had the same issue, it is most likely the passenger seat occupancy sensor. Search through the e39 forum and you will find it is a very common problem. There is also a writeup with a DIY fix, but it involves removing the seat and leather to access the sensor.