Paint and Body 2014 GT hardtop, water inside rear body panel

[Bcole42166]

I recently started noticing water sloshing in the rear panel behind the pass door and ahead of the rear tire. It happens after going thru a car wash or a heavy rain, usually goes away in 24 hours. Carpet appears dry. It seems there are many talking about this but no real direction on where I should start looking. Anyone found a source for this problem?