If you are still looking for an answer on this, the drain tube barely sticks out of the passenger side firewall where it's covered with insulation. The drain tube itself is very short and is likely missing the 90° rubber drain extension that channels water away from the firewall and down. I've had two of these vans with this problem. Water dripping out of the tube gets wicked back into the interior along the tube itself, right into the foam padding, from the force of air as the van is moving. From underneath, feel up the firewall until you get to the insulation and then feel around as you go up. You'll feel the drain tube. I used a PCV elbow I got at Autozone for $5 and glued it on. First I roughed up the drain tube using a tiny screwdriver so the glue had something to hold onto...tedious but worth it if you can do it. Here is one of many threads I found on this issue and the parts available to fix it yourself.
http://www.automotiveforums.com/t725..._in_floor.html
Just make sure you lift up the carpet and let all that water dry completely! Mine took 2 days in hot weather using towels and a fan. Prop up the rubber/foam insulator that goes up the floor pan incline to let that dry also. Good luck!