Is your diverter box door working properly? If it does not move with the control than it shuts the heater/defroster ducts off. I noticed that you had blocked the ac vents off according to your post and it sounds like the air flow may be directed to them instead of the heater/defrost.
Out of the (5) trucks with ac that we have, all (5) diverter box doors had to be repaired.
Jim
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