It is on the right side above the right engine. On the 717 it has been moved aft so it doesn't exhaust right on to the engine cowls and upper thrust reverser bucket. What do you mean by "normal" place?
I got ya. The DC-9 family has a tail cone that can be jettisoned in an emergency and a slide deployed. Therefore no place to put anything back there. The apu is in the bottom of the aft accessory compartment. Inlet on the bottom on the DC-9/MD80, above the left engine on the 717.