What was found
KBNA - THE CREW REPORTED THAT SMOKE WAS COMING FROM THE WINDSHIELD HEATER SWITCHES DURING CLIMB-OUT. THE SWITCHES WERE TURNED OFF. AN EMERGENCY WAS DECLARED AND THE CREW DONNED OXYGEN MASKS. THE AIRCRAFT WAS RETURNED TO KBNA AND AN UNEVENTFUL LANDING WAS MADE. MAINTENANCE FOUND A LOOSE TERMINAL SCREW ON THE WING INSPECTION LIGHT SWITCH/CIRCUIT BREAKER. THE SWITCH WAS REPLACED AND OPERATIONAL CHECKS WERE GOOD. THE AIRCRAFT WAS RETURNED TO SERVICE.