A/C Problems

Hello everyone. I was wondering if someone could help me out?.I have a 86 mercury cougar. When I turn on the a/c air is blown from the top of the dash but when i switch for the air to blow from the front of the dash it continues to blow air from the top of the dash board. In short whenever the a/c is on the air is not blown from the front of the dash but rather from the top of the dashboard. Anyone could help me out on what may be the problem here?..thanx alot.. and enjoy your cougars..