Good evening and thank you for inquiring with JustAnswer, my name is XXXXX XXXXX i will be the technician assisting you today. What i have seen on the '03 Stangs is the dain grommet in the front of the quarter glass pocket getting filled with residue and plugging up causing water to build up and over time slosh around and seap onto the rear floors. Beyond that there isn't much else that could be causing this except top leaks. I would first inspect the grommet and clean it as well as poke it through to ensure proper drainage. Then with all components apart watertest the vehicle to attepmt to duplicate the concern to trace the origin of the leak.
first remove the rear seat,
The rear seat cushion pad and frame (600A88) is retained in the following manner:
then remove the quarter trim panel:
inspect the grommet at the bottom of the quarter pocket on the forward side
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