Why is the ford fiesta heater not working on the blower?

If the heater is not working in Your Ford Fiesta then there are a few likely sources. The first is of course the blower motor. When the blower motor fails it will not be able to circulate air throughout the vehicle. If Your HVAC system is working and blowing cool air then it is likely not a blower motor problem. That leaves problems with the heater core or the thermostat. The thermostat can control where the coolant from Your engine will go and if it is not sending it to the heater core then there will be no heat in the vehicle at all. If Your heater core is leaking then that is a sign that it will need to be replaced since a seal on it has failed.