Yeah. I had that. But mine is school bus. The rear lights didn’t leak water but somehow dirt was forced in from underneath. I deleted the lights and foamed everything back there. IMG_1025.jpg
When I drive down a dirt road you wouldn’t believe how much dust is kicked up from the radiator fan. Then you have your exhaust. And since your rig has the aerodynamics of a brick, the turbulent air back there is amazing. You have a small hole/crack somewhere. Maybe spray that back wall w that stuff that you can make a boat out of a screen door.