I don't know about dogs but my entire family gets motion sickness (my Mocha used to but grew out of it...his human relatives are still suffering). What helps us is to always keep the A/C on (even in winter time) so the air is free flowing and not stuffy. None of us can get into a parked car without first "air out" the interiors. Also, we never look out the side window. When we speak, we all look towards the front and tell people sorry about not making eye contact because we cannot look back or to the side.
I found an article that confirms this for dogs also. Hope it helps.
Dog Car Sickness and Motion Sickness: Causes and Treatment