And by "on the back" he means as if you are looking from the rear of the car - forward, then it would seem to be on the back of the tank (from that perspective). Looking from the front perspective of the car, it is on the front the tank. Better said, it is between the rear end and the gas tank, but strapped to the tank.
And, jeez, I can't find a single adequate picture (Google images) to send him to. What is up with that???