If you're looking for a great way to keep your dog from getting your car all dirty, the Twist-N-Go Dog Kennel
by Ware Manufacturing is the only way to go. When you're not using it, it folds up to about an inch thick and can fit in your trunk.
When you are ready to use it, you simply pop off the elastic strap, and it springs back to it's full size as a kennel. It has saved the back seat of my car, and is the best dog purchase I've made to date! It comes in multiple sizes depending on your dog size.
You can get all of the specs as well as reviews on Petco.com