My light came on at about 66,000 and it was the driver side seat belt buckle. Cost $250.00 to have a new buckle put in. IMHO a real rip off for a $10.00 belt buckle. I took my old one apart and there are two sensors in it. One is a micro switch and the other is an infrared beam sensor that is blocked when you push the belt into the buckle. The infrared sensor either quit emitting a beam or the lens was dirty.
Try this and see if it works: soak a piece of old t-shirt in rubbing alcohol slide it over the seat belt latch and buckle it up so the wet cloth goes into the buckle for cleaning the infrared lens. Do it a couple of times and then see if the light goes out. If not then you have a trip to the dealer for diagnostics. Also the SRS computer is separate from the ECM and your ECM scanner will not work with it. Whatever you do, do not try to replace the belt buckle yourself unless you want the risk of firing the air bag.