WHILE TRUCK WAS PARKED AND SECURED, A FIRE STARTED IN THE WIRE HARNESS ABOVE THE GAS TANK. FIRE WAS CONTAINED BEFORE VEHICLE BECAME ENGULFED. FIRE DEPARTMENT AND POLICE DEPARTMENT REPORTS MADE. INSURANCE COMPANY INVESTIGATED. DETERMINED WIRE HARNESS OVERHEATED AND IGNITED GAS VAPOR LEAKING FROM PLASTIC FUEL SYSTEM LINES. DUE TO USE OF PLASTIC MATERIALS, FIRE SPREAD RAPIDLY, INCLUDING THE TOP OF THE FUEL TANK MELTING. TRUCK HAS BEEN REPAIRED.