Where is the APU in the case of an S-duct (like the L-1011)?
(Source via aircraft.wikia.com)

APU is number 134 on the cutaway, which you can find near root of left stabilator leading edge. It is ahead of the engine, below the intake duct, on the left side.

By Reedy (Own work) [CC BY-SA 3.0], via Wikimedia Commons

Its exhaust is on the right side close to the stabilator's leading edge.