M.W.
IF she is getting car sick which could be, You might try fastening her in the front eat on center seat belt. I am assuming because of we you might have a husband. You might try riding in the back with her. patting holding hand. But car sick kids do ride better in the front seat. My old 1965 car when my son was little had a center seat belt. I fastened his seat in the center when someone rode with me and in the passenger seat when it was just the two of us. She has to adjust to seat belts and carseats of some sort in the car.
Also if it is just a matter that she needs to lye down to sleep how about stapping in a small portable crib. I had one that was half the length of the back seat and about 3 ft wide. Fit perfect.
My son loved his car seat and would simply fall asleep in it I hope some of this helps M. W PS my child is grown now (31)