Ventilation problem

I own a 2000 Dodge Dakota Quadcab (94000 miles). I have a ventilation door problem. Any selection on the air circulaion control that should utilize the upper vent outlets causes the air to blow from the heater outlet. Sometimes, on the low blower setting, air will come out of the upper vent outlets until a higher blower setting is used, then it reverts back to the problem described above. Any ideas? Thanks.