Just spent almost $200 for a dog harness for the back seat of the Cayenne. It attaches to the seat belt and is tested to keep a 90# Golden Retriever in the back seat in a head on collision.
Found most of the other harnesses say they are tested, but don't actually pass. This one is made in Germany and is tested and certified to hold a specific max weight dog for each harness size.
Figure if I survive a car wreck, Pepper should as well. Some States are now requiring a cage or harness for animal travel.