We dealt with that for over a year. It was awful! Some kids really do much better in a convertible but it didn't help mine. Does she sit comfortably in the car seat out of the car? I wonder if its the motion causing nausea.
Like pp, we found loud music sometimes helped as did bright/flashy/noisy toys because he didn't have access to them elsewhere. With three older sibs, they could help with singing songs, itsy bitsy spider, making faces and playing peekaboo, or reading books. Mine even played catch with a small balloon or foam ball for a while. Any of that was safer than trying to concentrate with the screaming.
Ds1 (2006). Ds2 (2010). Ds3 (2012).