I have two yorkies, a yorkie poo, two maltese, and a cocker spaniel. All of them, even the maltese who are 12 weeks old have no problems in the car. They will lay down and go right to sleep the entire time. Just not Scampers.
I have been taking Scampers with me in the car since he was 12 weeks old, he is a year old now and has not gotten over his fear of riding in cars, I don't think he ever will. I do leave him at home more often but I still take him with me. Not nearly as often as I use to but every now and then I take him with me. I try to take him on shorter trips at the time so he isn't stressed out for so long. I just feel so bad that he is like this. He loves meeting new people and he is fine once the car is stopped. Pretty soon I might have to give him a tranqulizer for traveling long distances. Being stressed out for a long car ride is not good for their little bodies.
I really hope you have better lucky with yours than I do with Scampers. I guess some dogs will just never get used to riding in cars.