The passenger side would get very wet inside when it rained. I found a flat rubber drain on the cowl on the left and right side. The drain gets clogged with dirt and leaves and backs up the water to flood the carpets.
Squeeze the side to open the bottom and run your finger inside to clear out the crud. Problem solved!

 

We know...

 

I had to do that too, wasn't getting a leak but one day under the hood I noticed that drain and felt down into it, full of dirt, hosed it right out, never an issue since. Previous owner must have never checked it.