I just experienced the same problem. Turns out the drain for the vent area in front of the windshield (you know, the slotted area between the hood and the windshield) drains out a hole on the passenger side that's hidden behind the fender. This hole is above hole for the radio antenna wire where it enters the cabin. On my car, the grommet for the radio antenna wire hole is gone, someone had used some plumbers putty as a fix but it has been jostled around and was no longer fitting tight. So the water was coming out one hole, dripping onto the antenna wire, and following it into the cabin. During a particularly rainy period two weeks ago, I ended up with about an inch of water on the passenger floor. You can test to see if this is your problem by running water into the grilled area behind the hood, and meanwhile watching through your open door into the fender area to see where the water is going. You should also be able to see water entering into the cabin while you do this if this is your problem.
Hope this is clearer than mud. Good luck, and if anybody has any suggestions for a permanent fix please let me know. I put some new plumber's putty in but I can see already that its probably not going to last. Thanks.