hello, there is a tsb for this condition and it involves installing an oring on the a/c drain tube. they have a problem because the drain has a slot cut into it on the bottom of it and water from the drain will run back the underside of the drain to the firewall where it gets soaked up by the foam seal at the firewall. once that seal is saturated it will then leak inside the van. when you install the oring to slide it onto the tube and stop it just rear of the slot, that keeps the water from running back to the firewall from the drain tube.
ill get you the tsb number