My son had the same issue--and it was car sickness. While my daughter would drop off immediately, he never did, and he'd start screaming pretty fast, too. The only true comfort I can give you is that once the carseat is turned around (at one year) she'll be a lot better. But it's very hard.
Do you have a mirror for her, one that lets her see you? Any crackly sorts of toys/blankets/manipulatives? She's a little young for them, but as she gets older they may be able to distract her. My son often responded well if his sister rubbed his head, but every baby is different.
I feel for you. My son is seven, and much easier in the car, but he still rarely falls asleep in the car, even on long trips.