What's under the rear fender panels?

[QuinnyQuad]

My new GMC Sierra truck with Quadrasteer has the same bed as the Denali, which is described as "Wideside, steel with double-wall construction and flexible side fender extensions."  The entire side panel from the cab back to the tailgate is plastic (flare) and I wonder if anyone knows what it looks like underneath? is it rough framework or a finished "fleetside" fender which is then clad in the flexible fender extension?

???

[ToeJam]

<bump>... I'm interested in this too.