I don't know what to call this piece - it's maybe the C-pillar? It's not a pillar, though - it's the black plastic trim on the back door. This is the part:
I haven't really tried to do anything yet, but have been looking for something of a how-to...haven't dug around on the forums TOO much yet but maybe someone knows of something? The one on the car I bought is broken off and is causing the door to rust from the inside.