heater and defrost problem 1988 silverado

My 1988 silverado has a problem getting the air to blow out of the floor vents and the windshiled vents. The control on the dash shows it to be on heater or defrost but the hot air comes out of the AC vents only. Please help.

Check the door on the duct behind / underneath the glove box. I think on your model it is vacuum operated. It should transfer the air door to make the air blow out the bottom. Also check the cable or hose from the control to the door.

Thanks I'll give it a try.