Check the fuse link on the driver side strut tower.

If the air volume has decreased , but the fan speed has not, you want to change the cabin filters.

If the motor has decreased in speed, you want to check the brushs/bearings and regulator.

What have you checked??

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A Dalton

The links should help you understand, probably the regulator, if the motor spins by hand
http://www.peachparts.com/Wikka/W140BlowerMotor
http://v12uberalles.com/pollen_filter.htm