Right side, behind the rear wheel arch. The black path to the left is the mounting position for the rear bumper. The green-painted panel inside is normally between the petrol tank and the outer skin. Below/behind the green panel is the bottom of the body which is normally under the petrol tank. The hole in there (with the dirty rag hanging through) is where the drain in the bottom of the petrol tank normally falls through.

The new prefabricated panel above the arch is clearly visible.