I feel your pain!! Although my cavalier loves to get in the car. She jumps right in, even if she didn't I would pick her up and put her in. She might be feeling a little car sick, that's possible. Try to limit food and water before you put her in the car.
Try getting a 150 Newfoundland in the car that doesn't want to go, now that's a real problem....LOL . People said the same things to me, you probably only take her to the vets. Like you, that wasn't true I would take her to the park, to my Moms house and different 'pleasant' places, she just doesn't like the car at all. Someone has to help me get her into the car when I have to take her. It's very interesting.
Sharon, proud Mom of Scout (tri) and Breeze (Newfie)