Clip the rear seat belt belt into the socket, there is a very cheap collar/ harness attachment for the dog using the inertia reel obtainable from most pet shops which fastens to the empty fixed car belt and you just clip the dog to that. It cleverly allows some movement for the dog using the inertia reel of the car belt, enough to make the dog comfortable but in my experience they don't tend to pull at it and move out of position. Should the worst happen the inertia reel belt will operate in the normal way!