There are different ways to go about solving the issue.
Many times people will be rushing to get out the door. They put the leash on the dog and hurriedly put the dog in the car and that is really all the information they are giving their dog. The dog does not understand, that he is expected to be quiet in the car.
A few days ago I worked with a couple, whose dog struggles with this and they were getting nervous about going on a trip with their dog. My recommendation to them was, that while one of them drives, the other should be in the backseat and put the dog on a leash. It may be a bit of a challenge at first, but it will give them the ability to give valuable information to their dog.
Preferably, start by taking daily short trips in the car or even just out the door and up and down the driveway. Teaching your dog to be calm inside your home is one thing, but your dog can also learn, that the same calmness is expected when outside the home and inside your car.
Additionally, if you have a dog barking in the car and you are on your own, going through a drive through and picking up food is not a good idea, since a dog might become more aggressive, when food is involved. In that instance, the crate is your friend. To stop barking, I would also recommend the use of a bark collar or a remote collar.
Longoriahaus Dog Training is a dog training service in the Kingwood, Humbel and Houston, Texas area. Longoriahaus Dog Training can help you raise a happy and reliable dog.