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It is clipped, and the sill is clipped and has a weirdly shaped connector to hold on to the weather stripping, but once you get the sill up, the panel comes off easily. Here are some pics:
By the way, what's the little sensor under the sill there? (Two little green tips.)
Thats a dummy plug in the harness. I believe that harness is used for the lighted door sill plates.
Either that, or the auto power deployed running boards. There is a rubber grommet under the carpet near that connector. I used that grommet to bring power and coax into the truck from the engine bay.