1999 Dodge Caravan fan on heater and airconditioner

My fan does not work on the heat or air conditioner. When im on the road, the vent puts out heat and cool air enough to be comfortable, but turning the knob for the fan to blow does nothing extra. The only time the fan blow airs with the knob and puts out the air it needs to is when the van has been stalled a long time, parked some where for over a week, and then it will work for about 5 minutes then stop. Someone told me its a short in a wire, and cheap to fix, a spark plug or something, and garages tell me they need to fix the fan and the labor will cost about 400.00. What does it sound like to you?