Has your windshield ever been replaced, or has the wiper box/cowl trim been removed for any reason? I don't know about the newer cars, but on the GC/GD era cars, the cowl trim attaches to the body via a pair of pegs that go through the top of the cowl. There are supposed to be grommets in those holes. The passenger's side hole is directly above the inlet for the AC system, so if the grommet is missing or leaky, water will drain through the AC blower motor and soak the carpet in the passenger footwell.
I had an issue where my old '99 wagon would get wet floorboards after a heavy rain, and it was basically this. We fixed it through liberal application of clear silicone. It wouldn't surprise me if Subaru continued with a comparably dumb design all the way through the current models.