deleting
 

You might get more information in the GD thread; however, to answer your question, it should be bone dry down there. It sounds like something has compromised the integrity of your underbody in that area. It could be a bad weld or split metal or physical damage that has progressed to rust. If you're still under factory warranty and the source of the leak is not physical damage, have the dealer look at it. Otherwise, take it to a repair shop or body shop, or search for the source of the intrusion yourself and repair it. If it was a hatch weatherstrip, you'd also be wet in the cargo area above the spare tire compartment, so I don't think that's the source of your problem. Good luck, and don't let it go too long.

this happen to me the other day when i went through the carwash.

This keeps happening to my GD during heavy rains...yup, the last week.

Is your antenna leaking?