DESIGN OF AUTO HEATER MOTOR SPEED CONTROL IS DEFECTIVE. SPEED IS CONTROLLED BY SWITCHING IN VARIOUS WIRE RESISTORS MOUNTED IN THE AIR PLENUM WITHOUT A PROTECTIVE COVER OVER THE RESISTANCE WIRE ELEMENTS. RESISTORS GET HOT AT LOW MOTOR SPEEDS AND INCINERATE LEAVES OR PAPER IN THE AREA. THIS FIRE MELTS THE PLASTIC HEATER FAN HOUSING, AS WELL AS THE A/C EVAPORATOR. SITE OF ENTRY OF LEAVES IS UNKNOWN, BUT ONE DEALER REP SAID THERE IS A GAP IN THE FENDER WALL. TWO DEALERS' SERVICE WRITERS AND AN INDEPENDENT MECHANIC ACKNOWLEDGED SEVERAL SUCH FIRES IN EXPLORES AND F-150'S A PERFORATED METAL COVER OVER THE RESISTOR ELEMENTS IS MANDATORY.