you dont NEED seats. where to attach the harness to is the next question to be answered.
And where would a bar like this mount to? You have to have a proper roll bar to have a mounting point for a 4 point harness. I've seen where they put the other mount in the floor, but that also makes the strap very long.i think they sell a bar that goes across the car for a harness if you dont have a full cage.