Hi L.,
We went through this with our first baby. It took me until she was nine months to think of the motion sickness possibility, and that was it. Our pediatrician said at that age, we could try a little benadryl to treat the motion sickness - it did help but not too much. What helped most was if she had something to focus on - so I usually tried to hire a helper (yes, expensive) to sit in the back seat and play with her and keep her focused on something other than the things moving outside the car. It helped, but often I just sang her favorite songs for what seemed like forever, we came up with a version of Old MacDonald where we went through the alphabet and then did it all over again! :)
I'll bet a DVD player would work, we didn't have one at the time, but it should help keep eyes focused on one spot.
My DD did a complete turnaround when we were finally able to turn her forward facing - unfortunately, she didn't reach 20 pounds until she was 2, so it was a LONG two years.
We did spend so much time at home, going on walks, and trying to have playdates at our house. Driving was awful, and we were quite isolated. Also, all that could contribute to some post partum depression for mama, so help her look out for how all this effects her!
Best wishes.