No kidding. On most or all of those planes, with the APU Master Switch in Run or Start positions, the APU ram door opens downward before the actual start sequence begins until about 95 percent RPM, then it closes and the non-ram doors open, being flush with the fuselage. They are on the bottom at the rear of the fuselage. On the ground with the Ram Air switch on, the right pack switch (best used in HP bleed off) can push air from the tail compartment into the plane, but it is not cooled. Whether this technique means the air is filtered better compared to APU compressed air via the bleed air valve, maybe the mechanics/engineers know the answer.
I hate to think how much fluid from the two aft lavs could/might have dripped underneath over the last 30 years or more before it evaporated or froze.