I have water in my jeep?

[Elise B.]

I have a Jeep Wrangler '98 sport with a 4.0 liter engine. It works great except that the floor board on the passenger side keeps getting wet. I just noticed it and my uncle (who gave me the car) said it's done this the entire time he has owned it. It looks like there is a trickle coming in from a plug that wires go into but I can't be sure. And it's not a little bit of water, my floor mat was completely soaked. It wasn't my A.C. coolant nor my engine coolant, their levels have remained constant. I'm stumped, any ideas? Thank you so much for your help