There are a serious of self-tapping screws that attach to the rear bumper, inner arch and (I think) sill cover. You need to remove all of these and then have a fight with it twisting it out. You may find the exercise easier by removing the rear bumper. It's not necessary but every time I've had the liners out I've had the rear bumper off anyway.
If the rust is on the rear horizontal lip above the bumper that's where the outer wing has a couple of screws attaching it to the inner wing and is a known water trap. The mounting flange on the wing disintegrated when I removed the offside wing on my latest project car. The flange on the inner wing was untouched.