Best way to get my dog to travel quietly.

Our 10 year old Weimaraner is a terrible traveler. He gets very anxious when he travels with us in the car. He loudly whines/barks and jumps all over in the car. We finally got him a dog seat belt but he still is very vocal. We got him two years ago when he was left behind in a foreclosed home. We know very little of his history and if he was like this before. I feel so badly for him because he seems so upset. Normally he is mild mannered unless the door bell rings.