The mountings are different. The 9-3 one spreads out in a "Y" shape under the floor. The 9-5 bolts to the back of the bumper and has a rectangular plate under the rear of the car.
9-5 under rear of car (rusty version).

Bolted to rear of bumper (painted version!)

And the 9-5 has reinforcing brackets at the bumper ends too.