Brilliant!

Checked-out Amazon (why didn't I think of that?). This one...
Amazon.com: Deluxe Pet Car Seat Cover, Quilted, Water Resistant, and Machine Washable: Pet Supplies
...has dimensions that seem to be an excellent match to my back seat. Also has high ratings from a lot of people.

While of course my rear seat does not have headrests (remember, this is a 1985 M-B), I am confident that I can fit the elastic that would fit over the headrests to the brackets that hold the rear seat back in place.

Thank you for the help!