It is attached to the bottom of the fan box.
For my 2000 Silverado - I needed to remove the cover located on the passenger side below the glove box. This is the same cover that needs to be removed to access the cabin air filters. There were 3 screws. After the cover is removed you should see the blower motor resistor.
The heater blower resister is under the dash on the passenger side. It is inserted into the side of the blower motor case.
The year, make and model info would help but generally the heater blower motor resister is located close by the heater blower motor.
You have a bad blower motor resister..........
I found the resister, it is a small module held by 2 small screws on the base of the heater blower behind the glove box.
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 most likely will have a bad blower resister........
resister is located on the plenum next to the blower motor under the hood
should be the connector in the ac housing closest to the blower motor
how do you change heater blower resister merc a class
Check the blower resister
Replace the resister, if that doesn't work replace the blower motor.
The most common cause of the heater blower being on high only is the blower resister failing. The second most common problem is the blower switch going bad.