J.G.
My daughter did this for a long long time, she is 14 months old now and she still screams in the car seat once in a while. This is what I did that worked, I had in the front seat a lot of toys that I know she liked and when she started screaming I would start passing them to her. ( do it when you have a red light so I don't get too distracted). I also keep doing this until I get to my destination. This is very hard to do though and it gets frustrating sometimes but it is the only thing that actually worked and still does.
I would also try to get one of those hanging music things that you put on the sides of the car seat, so she can see it while you drive, those have music and it might distracted for a good 15 minutes. I would also suddenly point to the window and I would say: Look at that! A bird! (or whatever " a truck" "a car") and even though there's nothing there she would stop for a few minutes to wonder what I was saying, and I would do the sound, " look the train" choo choo choo!. But everything I have done has always worked for only a period of 15 minutes max. so you have to be improvising all the time. I also look a little crazy when I drive because of this but what can you do?