I don't have any problems with car sickness with Misha on short or long trips. Misha is the size of a min poodle and she is one spoiled girl, her car spot is on the back seat sitting on an old doona folded to the size of a large cushion and covered with a towel! She wears a harness attached to a lead thing which clips into the car seat clip. She sits on her throne like a princess and watches the world go by! On long trips she eventually crashes and sleeps.
I mention this because it might help settle your girl if she can see out the window. Maybe put some luggage on the seat and make a nest for her there?