Wet back seat and floor after rain

They make dye testing kits that help find potential leaks. Arrange the different color dye tabs around the windows, doors, and sunroof, then run some water over them. If one of the colors shows up inside then you know where the leak is coming from. Another potential water source inside the car is a missing floor pan grommet. You would either have to crawl under the vehicle or roll back the carpeting and padding to look. Water can splash up through hole to soak the carpet but only if the vehicle is being driven in the rain. More than likely you have a leak in a seal around a window or door.

It may be clogged vents that cause the water to backflow into car. I just had to have that fixed at the dealer. Apparently not covered.