I just fixed this exact problem on my wifes 2000. The problem was two clips that live under the wiper cowling. If it's only leaking in the rain or when spraying it this is probably the problem. I could recreate the leak and watch it come through the clips from under the dash.
I posted pics and description here. http://maxima.org/forums/5th-generat...ger-floor.html
* Remove the wiper cowl.
* Remove the two clips.
* Use RTV or similar product on the holes and replace the clips.
* Let the RTV set up before reinstalling the cowl because it clips into one of the clips.
Her car has been dry since I did this and we've had some heavy rain.