bad blower,buy a good one and replace
It is usually ( at least on a 2000 Ranger) locted on the blower motor housing in the engine compartment.
It is likely the duct work how ever it can be the fan has come off the fan motor
The resistors for the blower moter are located in the path of the air blown from blower motor
If you are referring to the blower motor speed, and only the highest setting works, then the problem is a bad Blower Motor Resistor Pack.
The blower may simply have burned out. If the blower is out then the heater probably still works.
Check for a bad blower resistor at blower motor
Loose connection or blower motor is bad
The blower motor resistor assembly is located to the right of the blower motor. The blower motor is located in the engine compartment on the passenger side of the firewall.
The Blower Motor Resistor Pack is defective. Replace it.
You need to replace the blower motor resistor located near the glove box.
when a blower motor only works on high, it is usually due to faulty resistor board. Should be located in the duct near the blower. the purpose of this is so the air blown by the blower keeps the resistors cool.
What has happened is your blower motor resisor has failed. When you turn the fan speed switch those settings are resistances that "damp" the blower motor and it runs slower. When this fails, it just has full current to the blower motor. Sometimes these are called block resistors. That likely is the problem.