I've gone through this a couple of times.
I followed a lot of the ideas from Dogs for Dummies - with some personal modifications.
For one dog it was clearly a nerves issue, the other it was excitement mixed with a bit of problem with curves (let's not talk about the Catskills adventure
![Confused](https://cdn2.able2know.org/images/v5/emoticons/icon_confused.gif)
).
In both cases, I started by simply going out with them and sitting in the car for about 2 - 5 minutes - and then going for a short walk or coming right back onto the porch. After that became easy for them, it was drives up and down the driveway - all of about 10 or 20 feet. Then a walk, or porch sit.
Then - down the block, park the car, walk home, sit on porch, walk down the street, drive home.
Then down the street to the nearest park, play, back in the car, home.
The process took about 2 weeks in total, with only minor regressions when things got too exciting for one of the dogs. No problems for about 4 years now.
In the meantime, I got used to covering the back seat and hatch with towels, so there are always towels for muddy or wet dogs after happy adventures. Both of them know to jump into the trunk with the towels if they're very wet - they get towelled off - then hop into the car.