the seatbelt is a very simple circuit. voltage travels through the clip and only gets the circuit completed when the male end of the seatbelt clip is inserted. the wiring has two paths. the path to illuminate the light has greater resistance than the path through the seatbelt. since electricity takes the path of least resistance, when the seatbelt is inserted, the current will only flow through the belt clip. check for broken prongs inside the female end of the clip. if they are fine, follow the wires to the PCM to find a break in the circuit. this is the only logical thing i could think of.