When my son was that age, he hated his carseat also. When we were in constant motion he would calm down and if we were lucky, he would fall asleep (he had colic), but as soon as we would come to a stop for a few minutes he would start to cry (unless he was in a deep sleep).
We found playing some calming music on the cd player in the car would relax him or even a lullaby cd.
Sometimes my husband and I would get so desperate that we would start singing.....anything......our national anthem, Take me out to the ballgame......any song we could think of...lol!
Every baby is so different, some babies are more difficult and sensitive to things, whereas some babies are so passive that nothing really seems to bother them.
This may be just the way your granddaughter is and this is perfectly normal behaviour for her.