I also have an E and it slides left a couple of inches and then lifts off. I doubt there's a different design for the C but anything is possible. It may be stuck if it's been left in place and cleaning fluid or beverages have gotten into the space between the seat and the cover. Did you try giving it a sharp shove from the passenger side of the car? You might also try tapping it about 3" in from each edge of the left seat (that's where the fasteners are located. And you might also grab the free end and wiggle it a little (left/right and up/down) while pushing.

Since getting my wagon about 2 1/2 years ago I've checked other wagons I've seen and not one has had that cover in place. I guess everyone just stores it unless really needed.

HTH and good luck.