From what I've seen most of these aftermarket AC units only use the inside air. Which is equivalent to the "max AC" setting in your new car.
With the AC on they dehumidify the air and reduce fogging.

If you still have a cowl top or side vent you can let outside air in anytime you want.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-