Intermittenly, while driving at night, the headlights on my 2004 Envoy XL simply shut off. I have tried covering the sensor on the dash to ensure no light is hitting it and it does not turn them back on. I also have turned the headlight switch to the ON position. The lights do not turn on. The small green light on the switch itself turns on but not the headlights. I am left with running lights only, but can also turn on the fog lights. Any ideas on what might be happening? I think I need a wiring diagram to diagnose. Again, it is intermittent so sometimes I can shut the car off and they will come back on. I am very handy with tracing electrical but I don't have the diagram. I am hopeful someone may have experienced this problem and can help me. Thanks.