Hi... I’ve been on this site since I bought a low mileage 2006 350z a couple of years ago. It had a musty smell when I first got it, but it was in great shape and hadn’t been modded at all. I started finding water behind the passenger seat after rain or washing it. I checked this site and others and didn’t find a cure. I then noticed water getting behind the driver’s seat. Turns out the drains at the bottom of the doors were clogged with dirt. There are 4 square drain holes at the bottom of each door. I flushed the doors out and I haven’t had water behind the seats since.

 

Troubleshooting leaks in these cars sucks... So many little areas leaks could be coming from!

Glad yours wasn't a PITA to find like mine was!

 

Thanks for posting. Finally someone with under 5 posts posting something meaningful for once; I'm sure this will help someone in the future.

 

Oh, it was a PITA... took me a couple of years to figure it out!