WAS DRIVING SLOWLY IN HEAVY TRAFFIC. WEATHER: APPROXIMATELY 50 DEGREES AND RAINING. I HAVE THE HEAT AND DEFROST (INCLUDING REAR WINDOW) ON. I HEAR A POP FROM THE BACK AND I THINK SOMEONE HAS JUST LIGHTLY REAR-ENDED ME. I LOOK OVER MY SHOULDER AND THE REAR PASSENGER SEAT, AT APPROXIMATELY THE RIGHT-HAND SHOULDER IS ON FIRE. I PULLED-OVER, TURNED-OFF THE CAR, AND TRIED TO DOWSE THE FLAMES TO NO AVAIL. ALL I COULD DO WAS WATCH IT BURN UNTIL THE FIRE DEPARTMENT SHOWED-UP ABOUT 10 MINUTES LATER. FIRE DEPARTMENT AGREED LIKELY CAUSE WAS REAR DEFROSTER SHORT-CIRCUIT. REAR SEAT AND REAR WINDOW DESTROYED. WATER AND SMOKE DAMAGE. CAR IS LIKELY TOTALED DUE TO AGE (11 YEARS) AND MILES (124K). *KB