PARKING LIGHTS TURNED ON THEIR OWN. WHEN VEHICLE WAS PARKED. PARKING LIGHTS AGAIN TURNED CAUSING A CAB INTERIOR FIRE WHICH MELTED DASH TO STTERING WHEEL, CRACKING WINDSHIELD AND BURNING CEILING LINER. LIGHT SWITCH ASSEMBLY NOW ON FLOOR AND WINDOWS SMOKED. NO HELP IN RESOLUTION FROM GMC TO THIS POINT. *JB