If will sleep a bit (ie 45 mins) plan pn lots of these - my little one is a 45 min napper anyway, but tires more easily in car, so can often do more of them.
She is also much happier with me in back with her, so she can hold me hand etc. This may help. Think of something baby likes (usually mummy), and accidental parent to your hearts content - this isn't an every day occurance. If will take bottle, feed to sleep (can bottle feed if you sit in back). Paci is a necessity if likes it - I allow to have whole trip if necessary.
Screaming baby in car is awful.
Oh, and do you have one of those CD that play sounds in the womb and household sounds etc? Didn't put my LO to sleep, but def calmed her down - try to get your hands on one of these - if not, try radio on static - it is white noise, which can help.
Good luck. Oh, and I also put lovie over face so she can't see, helps her fall asleep. A friend of ours once said that they read if you can get a baby to keep eyes closed for 3 mins, they will fall alseep - just gotta find a way to get eyes closed.
Again, good luck.