HN39;
Thanks for the reminder about Neofit's post, (#222) - I hadn't seen the photos.
I believe that the large dark pipe is pneumatic supply from the APU. The APU pneumatic bleed plumbing runs along the top port side of the APU and through the APU firewall (in the tailcone structure) and joins the top right-hand side fitting where we can see the APU pneumatic pipe (yellow capped), about ten inches in diameter. It is of the same "insulating" material as the dark pipe standing almost vertically on the starboard side, which we see in Machaca's linked photograph of the operator in the tail section. I think the small attached wire (left side of the vertical pipe in Neofit's image), to the larger pipe, carry the APU Bleed-air Duct Sensing Elements.
In Neofit's linked image of the aft fuselage, the fixtures at the mid-point of the two cross-frames are the forward fastening mounts/pivot points for the THS.