The little "grille" underneath the windshield wipers, which admits fresh air to the HVAC system and contains the wiper "transmission."

How or where does that drain rain away? To the sides, or are there drain tubes which might be clogged?

I'm getting wet carpets, the leak APPEARS to be under the dash near the steering column, and it does NOT seem to be the windshield seal.

They drain to the sides-take of the grill and inspect what is in it-could be related the cabin air intake(very common on the S30)or your wiper system attachment points. If you don't find the obvious, check out your upper hood seal(frequently removed/unintentionally manipulates)-and door seals, and the entire Windshield moulding(many ZXs have openings from rust in this area)-water can get to the carpet several ways

If you still don't find it, One dark night(or anytime in your dark garage) when everything is dry take the cowl grill out and put a bright drop light in the hollow it covers, disconnect the battery-or your interior light bulbs-then stick your head in the footwell and look for the light under the dash. It may not shine right in your face, but odds are something under there will be at least a little lit up and give you North Star to follow to your leak.

Will

Check all the rubber grommets that pass through the firewall (tricky to get to the ones behind the slave cylinders). Odds are you'll find one (or more) cracked-though a cursory visual inspection may not show faults. (I'm speaking from experience here)

Thanks for the good ideas.