When the AirBag light flashes, this is simply an alert that means "AirBag Fault". When the AirBag light is on solid, it's telling you there is a problem in the one of the components of the AirBag system. Unfortunately, there are littereally dozens of parts/sensors in the AirBag system (anything from an impact sensor to a problem in the seat belts, and anything in between), and to take a "guess" just isn't very practical. Your best bet as this point is going to be to get the "codes" read from the on-board computer. You will need to find a shop with a scan-tool that can read AirBag codes, as most local "Free" scans and "personal" scanners do not read AirBag codes. Most shops charge around $40-$65 for this diagnostic, but, like I say, it's the only way to know what part/sensor has the fault, and will give you the starting point for the diagnosis of the problem.