DRIVER'S SIDE SEAT HEATER MALFUNCTIONED CAUSING SEAT LINING MATERIAL TO IGNITE AND BURN HOLE IN SEAT LEATHER AND UNDERLYING MATERIAL. ALSO SCORCHED DRIVER'S COAT AND PRODUCED ACRID SMOKE. THIS HAPPENED WHILE DRIVING ON CITY STREET. DRIVER WAS WEARING HEAVY WINTER COAT AND SO DID NOT FEEL THE HEAT IMMEDIATELY.