replacing trunk extension

Most cases only the turned down portion of the trunk extension is rotted away. That portion can be replaced from underneath the car. Doesn't have to be factory stamped metal either. Most any sheet metal would work. It does enclose the truck.
It goes to the bottom of the quarter for 2 reasons,
So that panel doesn't flop in the breeze,
So they didn't have to weld it up high on the skin and hide those welds.