try only offering a certain yummy treat during the car rides. sprite is able to sit in my lap (and i know its bad...oh well) and she'll grab a bully stick and hop into the back seat of the car and just chew it by herself. and go somewhere totally fun...like a dog park. gruffi used to get it really bad till we started taking him everyday to the dog park. now he knows that whenever he goes into the car---its to go somewhere that's possibly fun. lol. like he doesnt mind sundays because we take him to petsmart for dog training.