Where does the APU draw its intake air?
The APU is usually located at the rear of the aircraft, in the tail, so the intake is generally somewhere on the side of this location. Aircraft place the inlet in different places though, and some even put the APU somewhere different. For some examples:


737 intake, the small NACA inlet at the bottom

E145 (the place they don't want you to put de-icing fluid)

B777 (the scoop that opens upwards)

A330 (a scoop on the underside of the tail)

B727 (inside of the wheel well)

Here is an interesting thread including other examples.