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,

Rear Seat

The rear seat cushion pad and frame (600A88) is retained in the following manner:

• Pins are located on the bottom front of the rear seat cushion pad and frame which secure the rear seat cushion pad and frame to the front floor pan (11135).
• The pins are inserted into locking plates on the floor crossmember with a push button release.

• The bottom of the rear seatback frame is retained with screws to the front floor pan.

then remove the quarter trim panel:

1. Remove the scuff plate.

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2. Using the special tool Torx45, remove the front safety belt anchor bolt.

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2. Remove the pin-type retainers.

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2. Remove the speaker grille.

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2. Remove the pin-type retainers and the quarter trim panel.

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2. To install, reverse the removal procedure.

inspect the grommet at the bottom of the quarter pocket on the forward side