I suspect it is rotten and there will be a part for it but my guess you will need to go to Ford for the part and will be quite expnensive, some Ford sill panels are not available and you have to purchase the whole side panel assy which is very expensive, personally I would cut out the rot,plate and weld as best you can,underseal and paint, the step is covered with the cover so it doesn`t matter what it looks like,good luck.
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