I do in fact have an idea! I just finished doing all of the metal-work on my 1990 2.3 and found that a popular place for rust was at the bottom corners of the front windshield. If you take off the fender, you will be able to see if there is in fact rust there or not.
What I found was a good sized hole, and through that hole would be the door pillar and passenger compartment where water could drain onto the floor potentially. I actually had to cut out quite a bit of the sub-frame to get at all of the rust

. However, a quick fix would be to just put a while bunch of silicone sealant in the area to stop leaks.
Also a question: how much water are we talking? half a cup or a rain barrel?
Generally if you haven't had a problem with rust yet then you should be fine.