I usually won't take my dog with me if I know the dog will have to stay in the car more than two hours. There's just not enough room to move around. Better to leave the dog at home. The added benefit is the dog may deter break-in. Buy a baby gate and set the dog in a place with tile or vinyl flooring then lay a blanket in the corner.

Another suggestion is to buy seat covers as the dog will remove any kind of sheets, blankets, etc. that you put down.