If the air bag light in on then you have a hard fault. you need to bring this in to a dealer.. Even if you used a real scan tool that can access the RCM I would not be comfortable telling you how to diag it..
It could be something as simple as the connector was left unplugged. BUT remember if you are going to check it disconnect the battery and wait 15 minutes before plugging in any air bag components.
BTW no matter what the mileage or age of the car is since you removed the air bag to paint it the warrenty on air bag components and wiring is now void.