I believe the hole at the bottom of the tail fin is for the
APU (Auxilliary Power Unit), The last time I saw an
MD-88, just after it pulled up to the gate and the engines were shut down, the APU was started and if you
looked carefully, at this "hole" you could see gas vapor
coming from it as well as the under portion of tail near
the rear stairs, where the APU is located. The system
enables the aircraft to have power while the engines are
not running, I believe.