My friend has a 2002 Sienna, and recently there has been a lot of water collecting inside it during/after a hard rain. He's pretty sure that it happens whether the van is parked or in motion. He's been inside the van while his son ran a hose over it for 30 minutes or so but didn't notice any leaks. Does anyone have any thoughts? Wasn't sure if this might be a problem that others had heard of before. Thanks in advance.....

I am having the same problem!! DId you ever fond the solution?

Yes, I also have the same water leak problem. Bought this van 2 years ago and immediately started to experience these leaks. After several trips to the dealer, could not find anything and extensive research on Internet, I found the following. On a different web site another fellow had the same exact problem. Here's the culprit, because most of the wetness is noticed in the fron (driver and passenger side) it gives the false idea of the whereabouts of the leak. It actually comes from the back where then it runs its way to the front and pools there (remember, the van is slightly inclined forward). The problem was the rear hatch seal. Apparently Toyota redesigned a new one. As we speak I am waiting from Toyota Corporation about this problem and if they can if fixed through the dealer.
PLEASE, do yourself a favor, call them (Toyota Corporation) and report the problem, apparently they don't know of this leak problem, but if enough people let them know, it will make much more impact as far as them doing something about it.
Hopefully this helps and good luck!

Do you all have sun/moon roofs?