What was found
CREW REPORTED, WHILE IN CRUISE, SMOKE APPEARED IN THE CABIN, ALONG WITH A ELECTRICAL ODOR. EMERGENCY WAS DECLARED, AFTER WHICH ALL ELECTRICAL EQUIPMENT WAS TURNED OFF. FUEL SUPPLY TO HEATER WAS ALSO SHUT OFF. AFTER SEVERAL MINUTES, THE SMOKE AND ODOR DISSIPATED, AT WHICH TIME THE NR 2 RADIO WAS TURNED BACK ON TO GET CLEARANCE FOR LANDING. SMOKE AND ELECTRICAL ODOR DID NOT RETURN. LANDING WAS UNEVENTFUL. MX INSPECTED THE ACFT, TO INCLUDE THE HEATING SYS. MX FOUND THE CABIN HEATER TEMP DUCT LIMIT SWITCH HAD FAILED (SHORTED). MX R & R THE DUCT LIMIT SWITCH, SYS CHECKED NORMAL. ACFT RETURNED TO SERVICE.