Wet Floor When Rains

About a week ago I was driving in rain/snow for about an hour and when I stopped I noticed there was a LOT of water on the drivers side under my floor mat. I thought maybe there was a hole in the firewall where they ran the previous owner's stereo cables through. Well, it rained again last night and when I got in it this morning to go to work, the driver's side floor was wet again. That ruled out the hole in the firewall since it got wet just sitting htere. It's right in the center of the floor board so I'm thinking it's got to be running under the carpet and padding from somewhere and collecting there in the center. Any ideas where this water could be coming from? Maybe a leaky windshield?