WHILE THE VEHICLE WAS PARKED, UNOCCUPIED AND LOCKED. THE FRONT PASSENGER SEAT HAD A CATASTROPHIC FAILURE OF THE SEAT HEATER SYSTEM THAT CAUSED THE SEAT TO CATCH FIRE. INTERESTINGLY, NO FUSES WERE FOUND TO BE BLOWN. THE SEAT CATCHING FIRE SPREAD TO THE PASSENGER DOOR PANEL. IT CONTINUED UP THE DOOR PANEL AND SPREAD TO THE DOOR TRIM, HEADLINER & VISOR. THE ENTIRE INTERIOR WAS SMOKE DAMAGED. THE FIRE DEPARTMENT RESPONDED QUICKLY SO THAT'S WHY THE DAMAGE WAS NOT WORSE.