The defrost function is the normal position of the door/flapper inside the plenum when no vacuum is applied to the dual-diaphram vacuum motor. That motor, located under the dash, has two vacuum lines running to it. One line is color-coded red....full vacuum to close off the 'defrost' air plenum and send air to the floor vents. The other line is color-coded yellow....partial vacuum when function control in in 'mixed' position(both defrost and floor). Check for vacuum to that vac motor. The vacuum resevoir is located (in my '81) on the passenger side of the engine compartment and resembles a 1lb coffee can cut in half. It has one vacuum line to it. That line comes from a small distribution manifold located on the fire-wall. Engine manifold vacuum comes into this distribution manifold. Between the distribution-manifold and the resevoir there is a check-valve....from that check-valve the line attaches to the funtion control connection block(the HVAC controls). Check for vacuum at the resevoir and after the check valve. If there is vacuum present, check for vacuum at the dual-diaphram motor. The 'weak' points are the lines, vac motor, check-valve. Unless the vac resevoir has somehow gotten holed, it should be ok.