VEHICLE WAS PARKED AT SKI RESORT SINCE ABOUT NOON/12PM. OUTSIDE TEMPURATURE BETWEEN 24F & 35F. STARTED VEHICLE ABOUT 4:20PM. WARMED UP ENGINE TO JUST UNDER NORMAL OPERATING TEMPURATURE. PROCEEDED TO PUT VEHICLE IN GEAR & TURNED ON VEHICLE HEATER TO 91F FULL SPEED (AUTO)CIRCULATE, FOLLOWED BY BOTH DRIVER AND PASSENGER SEAT HEATERS (TO LEVEL 3 OF 3 GREEN LIGHTS ON BUTTON). DROVE 15MPH SPEED LIMIT & AFTER ABOUT 3 MINUTES, LESS THAN 1/4 OF A MILE. SMOKE BEGAN FILLING THE CABIN, MEANWHILE PASSENGER PANICED SCREAMING I'M BURNING, I'M ON FIRE. IMMEDIATELY PULLED OVER & TURNED OFF VEHICLE, AIRED OUT VEHICLE BY OPENING DOORS, & INVESTED CAUSE. FOUND BURN ON RIGHT SIDE OF PASSENGER SEAT. PASSENGER RIGHT LEG THIGH WAS BURNED THROUGH SEAT & CLOTHING. VEHICLE HAS 230K ACTUAL OEM MILES.