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Vents are probably not the issue and plugging them will make closing the door an unpleasant experience. If you have a sunroof check the drains, but the most common leak in that area is third brake light seal.
I had a ranger once that the third brake light didnt leak but found water was coming in thru a 1/4 " spot at frame of back glass that factory didnt seal.Title of the thread is water getting in the rear seat vents and asked if he should seal it up so figured that was worth addressing.
Thats prob what ill doGive the seats a good scrub with carpet cleaner and a brush to clean them as deep as possible then get out as much moisture as you can before leaving them to dry somewhere warm and you can decide from there on the mold issue. Having the seats out and the back wall cover panel off should make it easier to see the source while its raining, somebody sprays the truck woth a hose or you go through a touchless car wash.