My thought would be the climate control push button module needs replaced. My 84 300SD was pumping hot air through all the vents even with A/C charged. Swapped out the button unit from my 85 300SD and cold air coming out of the correct vents.
I had to buy new unit for the 85 300SD after a bad auxiliary pump fried the unit so you might also want to check that if it turns out you need to replace the push button unit. I think I got my unit from
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Not an expert though.