I do a lot of heavy bodywork. Filling large openings is pretty straightforward if you're careful.
First thing to do is to make accurate templates of the openings you want to fill. I usually do this by applying masking tape to the opening, stretched tight, and cutting around it carefully with an X-acto.
Stick the pattern to a sheet of .030" or .040" styrene, and cut your piece slightly oversize. Then CAREFULLY fit the pieces you made into the openings.
Make them fit closely and try to get them to touch all the way around. Fit them ALMOST flush with the side that you'll see on the finished model.
Not a station-wagon, but I filled the scoops on Revell's Challenger I using the same technique.
The piece at the top of this photo has been glued along the seams with liquid cement. The one at the bottom has not. (You can run a bead of thick CA glue around the joint too, if you want.)
I generally use fiberglass to reinforce the filled areas. This is fine fiberglass cloth made for RC model planes, and at minimum, 30-minute epoxy.
That should give you plenty of strength to do your normal filling and sanding on the side that shows. I recommend using a catalyzed filler for that work, either epoxy based, or this Bondo brand "Professional" stuff. NUMBER 801. It comes with its own hardener.
Then just fill, shape and finish as usual.