Hi L.,
I know this is hard! My son did the same thing. He cried like he was in pain every time he was in his carseat. It wasn't until he was a little older that we realized he had sensory integration issues where your sences are over stimulated. (For instance some people can't wear turtle necks, because they would feel like it is choking them)His eyes were very sensitive to the sunlight and he was sensitive to hot/cold temperatures. It bothered him so badly that basically he couldn't stand it. He was never diagnosed with autism, but he had a lot of minor symptoms that would make us wonder. He did go through Occupational Therapy for a few years. Not sure if that helped or if he grew out of it. The carseat issue didn't last forever...he is now almost 8 yrs old, so I am not remembering exactly how long it lasted. Maybe your daughter just doesn't like facing the other direction, I can see how that would make her feel alone. Maybe just being held in the buckles is uncomfortable to her. I always had to sit beside my son when my husband was driving to 'try' to soothe him. The only thing that worked every time was singing him a jingle that he took to from a tv commercial. I believe it will get better with time, so you may have to bear with it for a while. In a few months she will be more interested in toys and able to hang onto them longer, that may help...that is when I am thinking my son started calming down.
Take care,
C.