Does she take a dummy, if so maybe one of those dummy clips so she doesn't drop it and can put it back in her mouth?
I found with DD if i put a light blanket on her and tucked it in tight she felt more secure.

Does she have a favourite toy? I found The little handle on the top of the door was great to hang something from, she couldn't quite reach it but gave her something to look at?

Sorry probably no help.. DD loves the car and falls asleep but these were things that worked when she was unsettled.