What was found
(CAN) DURING PREDEPARTURE GROUND RUN, CREW WAS ADVISED BY TOWER OF A SMALL FIRE NEAR NOSE WHEEL AREA.EMERGENCY EQUIPMENT RESPONDED AND WHEN CREW EVACUATED AIRCRAFT FIRE HAD EXTINGUISHED. MAINTENANCE FOUND COMBUSTION HEATER COMBUSTION AIR SWITCH CONTACTS CLOSED (NORMALLY OPEN) AND COMBUSTION AIR BLOWER FAN WHEEL DRIVE BROKEN OFF AND 50% OF WHEEL BLADES MISSING.NOTE: BLOWER FAN WHEEL WAS A REVCOR P/N Q300-187S PMA WHEEL INSTALLED. SUSPECT FIRE DUE FUEL FROM COMBUSTION HEATER OVERBOARD DRAIN IGNITED DUE COMBUSTION HEATER INCOMPLETE LIGHT UP DUE LACK OF COMBUSTION AIR. MAINTENANCE REPLACED COMBUSTION FAN BLOWER WHEEL ASSY. AND PRESSURE SWITCH. HEATER OPERATION CHECKED SERVICEABLE.