I'm assuming that is the front passenger side floorboard and dash area? Do I see a tinge of coolant in that water? If there is, more than likely the heater core, if not, I'm with SqOne, clogged drain.
Just a side note... On my Prostar, the heater core failed, then a few months later the AC condenser failed, call it planned obsolescence. All around that 800,000 - 900,000 mile mark. When I say failed, both developed a crack and leaked at the weld(s). Call it Michigan roads.
If one of the units are bad, from my own personal experience, I would replace both and get it out of the way. They are easy to do yourself, I would not take to a dealer or shop, they will rake you over the coals. The parts can be bought over at
http://www.truckersac.com. They use OEM parts in most cases at a big discount over the dealers.
Others may disagree, but with these older trucks, if you are in there working on the system and it is original, kill two birds with one stone. Unless, it is easy to get to both units and you do not mind tearing into the AC system twice.
On mine, it was kind of a job, not too bad, BUT a real PIA to do it twice, as both units are buried in the same area under the dash. If I would have known the future, I would have done both when it was apart the first time, along with new blend door actuators. You will need new foam too if and when you do this job, at least I did. I found new foam at Home Depot if memory serves me right. The foam is used to seal the box, it ages, fails and no longer does its intended job. I used the window foam with the sticky back, it got the job done.
Unfortunately these Internationals are not Rawze's old Toyota Tacoma. ;-)