my car has been having this same issue. We've have been getting rain for the past weeks solid and my car smells terrible. My passenger floor has been a swimming pool multiple times. Fixed the leaks today since we finally had sun and found a weird spot it was coming in. The frame rail in the engine bay butts up against the firewall,well theres like a 1/8th inch gap there and that was getting water in it and going to a pocket that had no drainage that i could see.
There was a about a quater sized rust hole in the floor and is was just letting it poor right into the engine bay. bad design from Nissan if you ask me.
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