Hi there.......this might sound kind of simple and strange, but I've heard this several times and half the time, it turned out just to be that the seat belt was too tight and the baby was uncomfortable. Try looking at the seatbelt and make sure the it is not too tight on the baby (you might already know this since you are an experienced mom, but you never know). Another thing, my daughter was the same way for a while there, it might have been when she was around three months. She got over it eventually. Just try making it a positive experience, say "yay, going for a ride" and smile real big........play some music so that she can hear it, make sure she has some toys hanging from the handles to distract her. Eventually she will stop, when my daughter went through this, it didn't last very long, now she LOVES going for car rides. I know its stressful, I used to get sooo upset and I never wanted to go anywhere because it was too hard to ride in the car with her screaming....but try to calm down and relax, the baby can feel your tension and anxiety, that alone can cause her to get even more upset.
Just some thoughts.....I hope it helps!
Good Luck, I wish you the best!