To the OP,
If dirt and grit is present in the bottom of the quarter panel where the water is collecting, then the rear tires are splashing water up onto an area with a poorly sealed seam.
The best way to locate the culprit seam is to dry out the quarter panel trough, jack up the rear of that side of the car, remove the rear wheel, spray water onto the wheel well area with a nozzled hose, and check the quarter panel for water. If it's present, then check the wheel well area for a poorly sealed seam.
If no water is present after spraying the wheel well area, then spray the area behind the wheel well where the gas inlet tube passes through the quarter panel. If that causes water to collect in the quarter panel, then check for a poorly sealed seam around the gas inlet tube(Note: A poorly sealed seam in this area was causing water to leak into the driver's side rear quarter panel on my 05 GT).
Do the same the procedure on the passenger's side.