J.M.
When my daughter was about that age and doing the same thing, we switched to a bigger car seat and found that eased most of it. She is big for her age, so I think she was uncomfortable. Another thing that helped is that I tried many soothing types of music and found one that she loves(took a while), but it's John Mayer-Continuum. I have no idea why, but she could be screaming her head off and when she hears his voice she immediately stops crying and calms down. I hope you find some suggestion that works. Good Luck!