What would cause water to collect on the floor board on the passenger side of a 2001 Sienna?

Could be the heater core leaking Could be a bad windshield or door seal Don't forget a possible plugged A/C drain plug. Does this happen when running your a/c?? If you have a sun-roof, the drains could be plugged. If so, they will leak in bad weather, causing water to run down the passenger door post. If the rain is hard enough, it will soak the passenger floor mats. I took mine in to Toyota and asked them to clean the drains (because I could not find a post telling me where they are located).