K.S.
I say if the child consistently goes in the carseat on trips to put her in pull-ups as a precaution. When she has a proven record of being able to control herself, wean her off of the pull-ups.
It really doesn't have to be an all or nothing prospect with the toilet training. Accidents will happen. If there is consistency with an accident in the car, then there is probably something about the motion or something else that she is not yet able to control.