Our vehicles are setup with Orvis dog seat covers for the back, and we use Ruff Rider Roadie harnesses that strap into the seat belt system. We have gone thru 2 of them over the last 11 years.
Why a seatbelt? Crates offer restraint/containment, but not safety for the dog. There was a video some years back made by a German insurance company. The plastic kennels explode in accidents when the weight of the dog hits the walls under force. That mirrors my experience when I was a responding EMT. Several calls where dogs were in the back of vehicles. Some contained, others not contained. Same result every time; they died.