A PLASTIC BURNING SMELL IN CABIN WAS NOTED AT HIGHWAY SPEED. UPON EXITING THE HIGHWAY, SMOKE WAS SEEN COMING FROM THE HVAC CONTROL PANEL. RECIRCULATION FAN WAS SWITCHED FROM HIGH SPEED SETTING TO OFF. THE SMOKE CONTINUED TO INTENSIFY, UNTIL ALL DC POWER WAS REMOVED FROM THE CAR. MAINTENANCE FOUND THE HVAC CONTROLLER, ROTATORY FAN SPEED SWITCH TO HAVE MELTED IN THE OFF POSITION DUE TO THE HEAT OF THE ELECTRICAL ARCHING IN SIDE THE SWITCH. DAMAGED LIMITED TO THE HVAC CONTROL PANEL P/N 1J0 820 045F MANUFACTURED BY VALEO. *TR