Here the best way to describe the flow. Fresh air goes in at the yellow arrow. It flows down the side of the cowl and out at the fan motor (round hole). From there, it follows the red arrow through the A/C or heater box and back into the cab at the rectangular hole.
If you have A/C and put it on max, there is a flap under the cowl left of the yellow arrow that mostly closes and the door in the passenger kick panel opens (photo #2) to recirculate air from inside the cab.
EDIT: I realized you probably meant the kick panel vents. I would guess the driver's side may have some debris in the cowl. I had a '64 Chevelle back in the day with no A/C and kick panel vents. Both vents flowed the same.
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