If the engine is at the 'normal' location of the APU, where did the designers of the L-1011 put the APU?
I did a search, and found this site which mentions
The APU is located in a compartment in the aft fuselage.
Another site mentions
Its still in the "tail". Its mounted below the #2 engine. Just forward of the tail skid.
But I don't see you could fit the APU in the small available space..