Your rear duels are enclosed in a four sided box, front and rear wall, top of the wheel well and wall next to chassis rail and all corners are sealed so water and road grime is confined in the enclosed area, usually has a mud flap on rear panel of box.
Evidently some are not and water is able to get into walls of compartments adjacent to the duel wheel well enclosure.
This would be the top of enclosure and walls would be attached off it to welded metal framing to make the box around your rear duels.