SMOKE CAME OUT INSIDE ON THE FRONT PASSENGER VENT, FIRE MELTED THE VENT COVER. TOOK THE GMC / BUICK DEALERSHIP, THEY SAID THEY CANNOT FIND THE SOURCE OF FIRE, SUGGESTING FIRE CAUSE IS EXTERNAL SUCH AS REFLECTION FROM THE SUN. DRIVER DOES'T SMOKE AND THERE'S NOTHING I CAN FIND IN THE VEHICLE THAT MIGHT HAVE CAUSE THE REFLECTION SUCH AS A MIRROR OR ANY SHINY OBJECT. THIS HAPPENED WHILE THE VEHICLE IS IN MOTION, CITY STREET DRIVING. VEHICLE TRAVELED APPROXIMATELY 2 MILES FROM THE HOUSE AT 8 IN THE MORNING. OUTSIDE TEMPERATURE AROUND 50'S DEGREES. I THINK THE PROBLEM MIGHT BE ELECTRICAL. THE AMOUNT OF SMOKE AND BURNING SMELL IN THE VEHICLE MADE IT VERY UNSAFE TO DRIVE IT.