I've never had to get an old dog used to it. But the puppy tricks are pretty easy to try. Start by just getting her in the car, sit in the back seat if you have to, play with her, just get her used to it. Then do the same thing, only this time turn the car on for a few minutes. Finally start to go for short little drives. Hopefully it will work for you as great as it did for me. All I need to do is ask if he wants to go for a ride, and he sprints to the car and sits at the back door waiting to get let in.