Well, In the image you can see the thread and nut of the eyebolt to the lower left. He is tied to it by a tether that is about 12" - 16" long. His blue leash is on the floor and you can see the white second tether and doggles for my other dog. I can bring them both but he is not such a fan.
Here you can see the tether..
Note that the seat cushion, just like the back, is removable, I leave it at home and he stands on the movable rubber mat that came with the car.