One problem related to defogger heater has been reported for the 1999 Lincoln Navigator. The most recently reported issues are listed below.
Spark plugs failed. As a result, vehicle jerked forward without warning. Coolant leaked twice in the past. The radiator assembly was reinstalled. The left front window squeaked. The trim on the front door panel was removed and reinstalled. The engine light had come on and the engine had run rough. The service dealer replaced the faulty coil pack on the #4 cylinder. The heater hose was replaced due to leakage. The heater did not blow air and displayed a burning smell.