This scrap of caulk-saver rod from the door gasket filler will prevent small pebbles & parts from jamming in the narrow gap, causing dents or rust. The bottom is open to allow water to drain rearward. It can also be used above the rear wheel arch reinforcements, and inside the door seals.

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Note that this '93 door skin does NOT have a side-impact beam ('95-up). It would run along the bottom outer skin about from the finger pointer past the missing paint to the back edge of the door.