Air Bag Light

98 Seirra,4WD, the air bag light stays on and once and awhile it goes out. The dealer tested everything and could not find the problem. Each componet was tested and all checkout fine. I was told that there was a contact point attached to the windshield that is part of the circuit. To the best I can remember, the problem started not long after the windshield was replaced. IS there a circuit to the windshield? Does anyone know of an elect. diagram for the air bag circuit? Thanks