There is a growing consensus that dogs should be restrained in cars...just like people are. There are many reasons why...dogs that are loose in the car can be a distraction to the driver and unrestrained dogs can be injured in car accidents too.
The American Veterinary Medical Association is promoting seat belts for dogs. Their campaign is called "Please Show Some Restraint". There is also a website called "Paws to Click" which explains how to install a Bergan Harness and the best way to "fit" your dog. Typically the harnesses fit around your pet's body and then attach to the seat belts or anchors in your car.
There is quite a bit of useful information on this site....http://airplaneapprovedpetcarriers.com/tag/avma/
Snoozer Pet Products makes the preferred safety seats that I prefer for my girlz while riding in the car...AND they can also be used for beds while in the house...or while traveling!! My girlz had absolutely no trouble at all adjusting to their seats and they LOVE them! As soon as they see their seats being belted in the car...in they go!