This really doesn't sound like the "infamous" leak, which affects the passenger side front.
There's two big plastic body plugs under the rear seat (about 10" in diameter) that go right through to the underside of the car (they're there to allow servicing the fuel tank sender/pump without dropping the gas tank). First thing I'd recommend is unclipping the seat bottom and making sure they're in place.
Second thing to check in that area would be the top sealing around the rear well, but that might require a pro to really check out.
__________________
2005 Mustang 4.0L V6 'vert
X-Charger S/N 004 (First X-Charged manual, First X-Charged 'vert)
|