Hello,
My 8 week old baby will sometimes have fits in the carseat as well, which I feel for you because it is very stressful. I have tried a couple of things that worked.. first I try the pacifier, he is in the middle in the back so I can just reach behind.. not too safe, but he will take it quickly if he wants it. I also put on the Mozart for Babies cd and that seems to work, it does need to be pretty loud to get his attention. If nothing works, I just have to let him cry and will "shush" him and after a few minutes he will calm down..
And just yesterday, I realized that the straps on the carseat needed to be moved up and since we moved those, her first car ride since was quiet!!
Good luck.