I'm having a similar problem after a decent rain, but mine is the floor in FRONT of the passenger seat. I have weathertech floormats that hold the water, but I have never seen where it is has come from. The water literally sits in the center of the mat, with no evidence as to where it came from. Top doesn't seem to be leaking at all, the area around the door is dry,….only thing I can think is it may be coming in behind the dash and dropping straight down onto the mats. I felt all around and looked as much as I could, but I'm still perplexed.