JB the box in your diagram seems to be the same as my 72 #2636. With a fresh air intake on top under the wiper grates. What do you think? **Edit; or is the air intake at the end of the fans, and the top of discharge .
Rene, it is a lot easier for the HVAC system to "condition" a closed system (X cu. ft of air recirculated) with AC
Adding a constant supply of fresh highly humid air should only aggavate the problem , the cold humid air will flow through the 190*+ degree heater core and turn into heavily moisture saturated hot air. (Simular effect to when you see your breath when it is cold) to cure this the air is "conditioned or dehumidified" by the AC core before hand. Regretfully , I suspect adding a fresh air vent may make it worse.
When I was young I had a butt ugly Mazda station wagon (non working AC) that would ice up the side and back window on the inside, while you were
roasting from the hot air blasting in your face. I cured the problem by buying a Toyota. I guess I could have fixed the AC and still be driving that beast.