I have a 1999 ford mustang. The heater works, but when I turn on the fan, the air turns cold. The air is hot if I do not turn on the fan.
Is your system HVAC or just a manual controls?
So you either have 1 of 2 problems, the heater core is plugged to point the any air flow cools it off so much that it won't radiate heat or the air blend door is not closing all the way in the heat position so when you turn on the fan it is drawing in outside air around the door inside the heater, the problem is that both problems will require that the heater box be removed.
Looking for a Used Mustang in your area?
CarGurus has 41,399 nationwide Mustang listings starting at $900.
Search Ford Mustang Questions
Ford Mustang Experts