What was found
UPON DESCENDING OUT OF 4000FT ON AN EXTENDED DOWNWIND FOR RUNWAY 4, COPILOT NOTICED FAINT ODOR EMANATING FROM REAR OF CABIN, AND BELIEVED MAYBE A PASSENGER HAD LIT A CIGARETTE. AFTER TURNING BASE TO FINAL AND RECEIVING CLEARANCE FOR THE VISUAL APPROACH ON RUNWAY 4, AIRCRAFT WAS ESTABLISHED ON THE LOCALIZER AND WERE APPROXIMATELY 5-6 MILES FROM RUNWAY WHEN DENSE ELECTRICAL SMOKE BEGAN FROM UNDER THE GLARESHIELD. OXYGEN MASKS WERE IMMEDIATELY DONNED, AND ALL FANS AND AVIONICS SWITCHES WERE TURNED TO OFF POSITION AND THE MAIN BATTERY SWITCH WAS MOVED TO THE EMER POSITION. BLEED AIR SETTING WAS SELECTED TO AID IN CLEARING THE CABIN. COPILOT INFORMED TOWER OF SMOKE IN THE COCKPIT, AND AN EMERGENCY WAS DECLARED ON OUR BEHALF, AND WE REQUESTED THAT EMERGENCY VEHICLES TO BE DISPATCHED. THE SMOKE INTENSITY SUBSIDED UPON FAN AND POWER DEACTIVATION, AND A SUCCESSFUL LANDING WAS MADE ON RUNWAY 4 AND THE AIRCRAFT WAS TAXIED TO THE FBO, WHERE PASSENGERS WERE IMMEDIATELY DEPLANED SAFELY. ON THE GROUND THE CABIN FAN CIRCUIT BREAKER WAS FOUND TO BE IN THE POPPED POSITION. TECH TRAVELED TO GSP AND INVESTIGATED ISSUE AND FOUND THE DEFOG FAN MOTOR BURNED UP CAUSING BREAKER TO POP AND FILLING CABIN WITH SMOKE/SMOKE SMELL. R&R DEFOG FAN WITH SV AR P/N: 5703-4, S/N: 1650 USING THE CESSNA 550 MM 21-21-01 AS A REFERENCE. OPS CHECK GOOD THIS TIME DATE AT WHICH TIME AIRCRAFT WAS RETURNED TO SERVICE AND RETURNED TO LYH.