Im not sure exactly what your problem is here, but the heater control box is very simple, so there could only be a couple of places to look. Make sure both defrost vent tubes are attached properly on each side of the box, and extend and attach properly to the outlets right below the windshiled vents. If they have come loose, you would get cooler air as they are venting under the dash, not on the windshiled. Also make sure your selector knob/lever is working properly. The lever activates a metal flap in the bottom of the box. Sometimes they get stuck and will not move properly. The flap may not be extendeding to a full defrost/closed position. Also check to make sure you have a good seal coming through the tube from the fan to make sure there is enough airflow to make it to the windshiled.. Hope this helps.
Mike