My penger side floorboards are wet, any ideas?

I have a '91 Mercury Tracer that I bought February of 2010. I had the engine replaced last summer, and the car has been pretty good ever since, until now. I noticed my floorboards being wet on my penger side near the end of winter, but chalked it up to melted snow off of my pengers shoes. I thought nothing of it until after spring cleaning my car. I scrubbed the interior carpets to get ready for summer, and when I put the car back together the mats and carpet were just slightly damp. Next day it rained. floor mats still damp. Three days of straight rain later, my floorboard carpet and floor mat are sopping wet. It is the most wet under the floor mat, up the side under the dash, and even under the seat itself. I know it isn't from the window, and it is most definitely water, so where could it be coming from? More importantly, how do I fix it?