your heater resistor is on its way out ..some are built into the heater motor some on a separate board depending on the make and model of vehicle
If its the fan speed your talking about its likely that the speed controller on the fan is broken and had its days
If you mean the fan - replace the fan speed resistor if the fan only works on high speed. Please supply more information - no heat or cooling? Does fan work? Does mode switch work?
Blower motor speed resistor is partially burned out.
do you mean the speed of the fan? if so, it'a relay generaly near the motor of the fan. If it's the heat who only came at high speed, you need to add some antifreeze.
The 2000 Mazda Protege has electric fans, hence there is no fan belt to change.
try replacing the blower motor resistor
If only high speed is effected check the high speed relay and/or the fan switch/electrical connections for problems.
Check the low speed relay and the resistor at the motor
One of your capacitors in the fan is not operating. I had the same problem. If you still have the high speed working on the fan there is a work around. If it is the high speed capacitor that is gone, take the fan apart and use one of the other capacitors to replace it. Use one that is closest to the rating of the high speed capacitor. In the high speed run the fan will not be using the internal fan capacitors for the lower speeds, so cap the wires off with wire nuts that come from the fan motor to the capacitors. Move the connections around until you get the high speed position to work. This work around requires that there is a wall switch that operates the fan. A Lutron model number DVFSQ-F will work very well with the fan. There is a toggle on the right side of the switch that selects the speed, low, medium and high speed. Depending on the position of the slider switch, that is the speed the fan will start in. The new fan switch is wired just like an ordinary light switch.
replace the blower motor resistor
More information would be needed to diagnose your problem more accurately, but here are some tips to start: 1) Fans will not work with dimmers. If you have a dimmer controlling your fan, it will only work when the dimmer in is the full intensity position (if at all). You will need to replace the dimmer with a fan speed control. If one switch controls both the fan and the light, this will not solve your problem, as a light will not work with a fan speed control switch. You will have to settle with a standard single pole switch, a remote switch, or get one more wire between the switch and the fan. 2) If you have a fan speed control switch, but the fan doesn't seem to respond as it should, you may want to pull on the pullchain directly on the fan. If the pullchain is not set to high, your speed control will not work correctly. 3) If #2 or #3 don't apply to you, and you are simply using the pull chain on the fan, you may have a bad switch inside the fan housing, loose or disconnected wires to the fan (unlikely), or simply a bad fan motor.
Its probably a bad resistor.