Dust usually penetrates from openings in the bed (stake pockets and underside of the rails) and around the tailgate. I tried taping up all the openings in my bed once and using a tailgate seal. It worked for a while but wasn’t 100%, and the baked on tape residue was very tough to remove. Water intrusion can be from a poorly sealed window or window frame. I had a shell that leaked around the frame. Get in and have someone hit it with a hose until you find the leak and it is usually pretty easy to seal it up.
