You might have a build up of leaves and dirt in the plenum chamber causing an overflow into the fresh air inlet, have seen that a couple of times. Failing that look at all of the common areas, around the heater box, the guard bolts that are internal in the cabin must have a good seal on them. Sometimes the drip check can get old and crumbly along the front edge of the firewall and can leak into the footwell. I know this sounds odd, but is there any water being retained in the closest door to where the water is getting in?