Three problems related to defogger heater have been reported for the 1998 Ford F-150. The most recently reported issues are listed below.
Heater core leaks needs to be replace 1 to 2 times per year, turn on the defroster and it will fog up windshield for you can not see the road. Per Ford dealership it due to poor parts, but the consumer needs to pay for it or get killed in a traffic accident if the consumer need to use their defroster.
The heater core failed three times within the last year. When the heater core failed, the antifreeze leaked and fumes entered the passenger's cabin. The manufacturer was notified, but would not warranty the part.
Heater core went out at 58,000 miles. When turning on h heat it would produce a gray whitesh mist coming from the vent. He took truck to the dealer, and dealer wanted to charge him $50. 00 for the part, but only $500. 00 for labor.