Hi Sarah, I would recommend talking with your vet about medication for car sickness. It can be helpful to rule out car sickness first before going the training route.
I am not a trainer, but my dog was the same. I got in the back with her and a treat. I sat there for 20 minutes with her without the car running. I got out with her, walked her around the car, then did it again. She is great now!
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