Like Kelly said in her response, I too usually am home when the kids are napping. My 8 month old still has 2 naps sometimes 3 so everything we do is usually centered around his nap schedule. But I do know that there are times when this just is not an option, when you have to get out for whatever reason and the baby ends up taking a nap in the car. Or you might be a person who does not center their schedules around the babys naps (that is a personal opinion and does not make you a bad mom/sitter, because you are doing something differently than us).
When you read my response just remember that screaming can be extremely dangerous in the car. Especially if it is raining or heavy traffic, it is a major distraction and personally I think you need to get it under control....so keep that in mind, my above feelings when you read my response. I veiw screaming in the car as dangerous as if your child was to step into the street. Also this is not how we discipline in the home, this form of discipline only takes place in the car.
This is probably not the most popular advice, but again, this is what I do with mine (and I have 3 that are 16 months apart-starting at age 3). Needless to say not only does a scream come out once in a while, but the carseats are all right next to one another, so I have them taking toys from one another, stretching one anothers arms, and every once in a while someone will stick their foot in Tino Giovanni's face since he is in a rear facing carseat! So I don't have time to pull over and talk to them or anything like that. I hear what is going on, or sometimes see it because I either glance in the rearview or am the passanger in the car. They get a warning which goes like this, "There is no screaming (or whatever the wrong behaivor is) while I am driving. It is dangerous to scream when I am driving (or "do you think it shows your brother that you have a loving heart when you pull on his arm? I don't think he likes it, he said stop.") If you do "this behaivor" again then you are going to get a mommy pinch! Do you understand?" I wait for a response, if I don't get one, I tell them that I asked them a question and that they need to answer me because they need to show me they respect me. They answer and I ask my oldest (who can talk extremely well) to repeat what will happen and my 2 year old (who is just learning to talk) I will just repeat it one more time and ask him if he understands again. This way I know that they heard me and understand what is going on. If I get a no I repeat again....anyway, there have been times when my daughter laughs at me and does it again (she is a very strong willed little kid who loves to see if I will follow through), or they just want to do it again....whatever the reason if it happens again they get a mom pinch....the mom pinch is just the quick grab behind the seat and get their leg or foot and pinch (not a really hard one). Once they get the mom pinch I remind them that I love them and that they got the mom pinch because they had disobeyed and done whatever the deed was.
Again, this is probably not the most popular opinion or discipline action, but it works. My children are not perfect, but they do respect me....and they also do not go around pinching or hitting other children. I do not believe it teaching children to pinch, if you have told the child that this is their consequence for their action.
Good luck.