It sounds like you may have a problem with the drivers seat belt buckle. There is a switch in the buckle which tells the air bag computer that the belt is properly buckled. The first step would be to see what fault codes are stored in the air bag control module. The code(s) must be read with a scan tool. The code(s) will indicate a specific circuit or circuits which are causing the fault. It would probably be best to have a qualified technician check out the air bag system on your 1999 Buick LeSabre.