Above the passenger side footwell you will find a white connector with 4 wires. Disconnect the plug, remove the two small hex-head screws and you can remove the fan motor resistor.
It is poorly made and may be corroded or burnt out, but it is certainly easy to remove and replace.
Nearside front wheel bearing
It would have a high speed relay and it would be in the power distribution box under the hood. The blower speed resister is for the lower speeds only.
navara's meter shows 220kmph, but it reaches maximum at 150kmph
The heater motor speed resister is burned out.
Most likely the resister is burnt out. When you use less than full blower, the blower speed switch shunts the 12v power though a resister to drop the voltage to the blower motor, slowing the blower speed. When the multi-speed resister is burnt out, it is, 'open circuited'. So no electricity can pass though to the blower. When the blower switch is on 'full', the resister is bypassed, allowing full power to the fan motor for maximum speed. It should be located under that dashboard in most vehicles. Just trace the wires from the switch to the resister. Warning: a working resister can get very hot when power is going though it.
I'm not a mechanic, but I do believe if you bypass the resister you will only have high speed on the fan. To that end, if the resister is your problem you should already have only high speed on the fan.
The most likely problem with the heater blower on your 2003 Sable is the blower resister. The resister can go bad and cause you to only have one speed.
You will most likely have a bad blower resister..............
Blower speed resister is burned out.
where is speed sensor on 2000 nissan altima What are the procedures on installing a speed sensor on 2000 nissan altima
Probably your blower motor speed resister is burned out.
where is 2005 Nissan speed sensor located