Sid,

I had a small dog who would always tremble in car rides, but she was so devoted she traveled almost the whole way across North America together one summer.

It turned out that the reason for the trembling was a tiny electrical leak in the cars electrical system that caused a constant current at low voltage. Not enough to be a short circuit, but sort of like those electric muscle stimulators that are supposed to help you get a flat stomach by causing muscles to tense and relax. (they don't work as far as I know, but that's an off-topic story).

I never did find out what caused that, but the problem went away with the car, when I got another car.

If you haven't changed cars, or don't have a second car to test this potential problem, try taking your baby in a neighbor's car and see if the problems persist.
Anyway, when I got a different car, she no longer had a problem.

Sincerely,
Stafford "Doc" Williamson