|July 4, 2011 - Removing the side panels in the back revealed this secret smuggling compartment.|
That's the outer skin you can see, along with the backside of the welds for the rear flares. A previous owner had cut out part of the inner panel in order to fit a speaker as well.
Seems to me this is a good spot for exterior noise to get in. You can see the large amounts of factory sound deadening.