I have used rear seat covers like these for at least two (often wet and muddy) flatcoat retriever hunting companions. The covers are basically a large rectangle of waterproof fabric with nylon straps that loop over the front and rear headrests. Do a web search for "dog sling car seat cover"or similar. I like the ones that do not have a cut out between the front seats since it keeps the wet dog from visiting you up front.
They are quick to remove when you want to load humans instead of dogs, and easy to hose off. Pro tip: Just be sure to chose one made of a fabric that is not super slick (dog slides from one side to the other when you take a corner) Cost between US$50 and $100.