Car seat problems

Hi I am a grandma and my granddaughter who is 4 and a half, non verbal and Autistic  has just started to become very agitated whilst in the car seat. This is a new thing as she generally is OK, we bought a bigger seat for her age but she nearly gets out of it and is putting herself at risk and everyone else. Tried lots of things but at a lots, bought gadgets. Any ideas that might work even for short journeys.