WHILE DRIVING ALONG AT 45 MPH THE FRONT DRIVERS SEAT HEATING ELEMENT WAS ACTIVATED. IT WAS SWITCHED ON TO ITS HIGHEST SETTING. IN APPROXIMATELY ONE MINUTE THE FRONT SEATBACK LEFT SIDE BOLSTER CAUGHT FIRE BURNING A DIME SIZE HOLE AND BURNING THE FLEECE VEST WORN BY THE DRIVER. THE VEHICLE WAS IMMEDIATELY SLOWED TO STOP ALONG THE HIGHWAY SHOULDER AND THE DRIVER EXITED AFTER TURNING OFF THE DASH SWITCH. THE SMOLDERING STOPPED. THE SWITCH HAS SINCE REMAINED IN THE OFF POSITION AND AN APPOINTMENT WITH A MECHANIC WILL BE SCHEDULED. *TR