The fan control module in the engine compartment has gone bad.
My 1994 did the same thing.....the problem was in the high/low switch....
Bad blower resistor. I just replaced mine.
No one knows if:The heater is NOT blowing - or blowing on high speed only, or... The heater IS blowing, but the air is NOT hotEither way, the "Related Questions" below will get you started
how is it my heater control on my ford fiesta flight 1998 only works on no. 4
Heater is not working in a 2002 ford foucs only work when fan setting is place in?
Defective heater resistor pack.
Sell your Ford junk its not the only problem you will have.
The most common reason the heater is blows cold air on a 1999 Ford Winstar is a bad thermostat or blocked heater core.
The heater motor speed resister is burned out.
Normally when a heater blower motor only works on the High setting, it is caused by a defective Blower Motor Resistor Pack. The fix is to, Replace it. In very rare instances it can also be a defective blower motor speed control switch, but rarely.
Possibly
Defective heater resistor pack.