I had a similar problem with my 2014 mustang
. My car sounded like there was water in the back panels and doors. I was not happy to say the least. I found there were black plastic rectangular covers on the inside bottoms of the doors. For some reason they seem to cover up drainage holes for water to escape. Ever since I removed them I never herd water sloshing again. I don't know if this is your problem but it's worth a shot. Hopefully it's not something more serious. It might be in the doors because mine actually sounded like it was more in the back as well, but this seemed to work.
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