EARLY MONDAY MORNING ON 12/17/18 WHILE DRIVING FROM ORLANDO,FL TO CLEARWATER,FL ON A MAJOR HIGHWAY MY WIFE HAD TURNED ON THE PASSENGER SEAT HEATER. NOTHING WAS OUT OF THE ORDINARY UNTIL WE SMELLED SOMETHING BURNING, THINKING IT WAS PROBABLY COMING FROM THE OUTSIDE. THE SMELL DIDN'T GO AWAY SO WE TURNED OFF THE A/C AND BOTH DRIVER AND PASSENGER SEAT HEATER UNTIL THE SMELL OF SMOKE WHEN AWAY. WE LEFT THE SEAT HEATERS OFF AND LEFT ON THE AC. WHEN WE GOT HOME WE STILL DIDN'T NOTICE ANYTHING UNTIL LATER ON IN THE MORNING WHEN MY SWEATER HAD A BLACK BURN MARK. I WENT OUT TO THE CAR AND NOTICED THAT THE PASSENGER SEAT HAD A BURN HOLE CLOSE TO THE BACK. WHEN I TURNED ON THE CAR AND TURNED THE PASSENGER SEAT HEATER ON I NOTICED SMOKE COMING UP AND THEN A SPARK AS IF IT WHERE A LIGHTER.