This is such a common problem, company i work for has 4 C15 vans and although the heaters work, the blower in every one has this.
The blower box is located under the dash drivers side, sometimes tapping it will start the fan. You can remove the parcel shelf and locate the blower box, remove a couple bolts on this to access the fan motor, it could just be that the contact brushes need a "gentle" clean. remove any debris and dust from the motor.
More answers here where i found this info; http://www.howtomendit.com/answers.php?id=100230
Mel
December 2010
Automatic Answer
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