Yup, thats how my drivers side floor and rocker panel rusted out. There are drains at the bottom of the cowl behind/under the rear of the front fenders. But leaves, twigs, dirt and small animals fall into the cowl through the vent and block the drain holes. Then when it rains, it fills with water, and your floor gets wet.
Now, how to fix it. You already have the kick panel off, now remove the cover or vent behind it. Mine is an A/C car, so it has a cover where the cowl vent would be on a non-AC. You can reach down into the bottom from there and clean it out. I drilled bigger drain holes and POR'd the whole inside on both sides.
PAWN SHOP FIND
RESTORATION IN 1980...
"You can have it all, my emipre of dirt. I will let you down, I will make you hurt"