There are a cople more areas it could be from. One is a rubber plug behind the kick panel on the passenger side. It seals a hole that goes into the cowl drain area. I found mine was leaking. This is a pretty good size lug. You can tell if it was leaking as soon as you pull the kick panel. If it has then seal it up with silicone or butal rubber.
The other area, and this may sound like it can't cause water in the front floorboard, Is the cargo light above the rear window. Mine was leaking bad and caused water in the front. The water comes in and migrates around the window and then down to the floor where it works its way to the front. My truck has carpet on the rear wall so I couldn't see where the water was going.
Do a search as I know there is lots of stuff on this item.