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If the fan is operational, you more than likely have an issue with the blower motor resistor. The power going to the fan is always the same, so the system uses a resistor to lower the voltage and therefore control the fan speed. Each setting has a specific resistance that allows the fan to spin at a certain speed, with no resistance at the high speed.
You can try to apply direct power to the fan and see if the air volume changes. If it does then you more than likely have a bad blower motor resistor. If not then replace the blower motor or look into the dashboard for restrictions.
I have included the inspection procedure for the blower motor resistor:
If the blower motor resistor does not match the resistance values then replace the blower motor resistor.
Please refer to the picture I included above.
The blower motor resistor is located on the box where the blower motor is located.