Oh my gosh, did we ever have the same problem. It was HELL driving anywhere. We even had toget oue then-5 year old special headphones to wear to block out the baby screaming next to her. I thought my baby must have been in pain, the screaming was so intense and would not stop until she got out of the car. But then I realized she would stop as soon as she got out of the car, not the carseat; she just had to be out of the car. Weird.
We tried toys, music, stories, mirrors...everything we could think of. The only thing that would keep her from screaming her head off during a car trip was my husband riding in the back with her, singing a song he made up that went
"Meara moo moo moo, she did a poo poo poo out from her bottom. It smelled like (eldest daughter inserted something here i.e roses, bathtubs, Nana, etc). She's always in an eating mood, she likes to eat the grownup food; she likes to eat her mom's ice cream and when she doesn't she likes to sreeeeaaaam." Verse after verse after verse, until we arrived at our destination. Any other song, any other person singing it...no good. That song, sung by Daddy and absolutely nothing else.
Thank goodness, she grew out of it after a couple of months. I still hate that song.