Looking at page 47 in our wiring diagram, which can be found here
http://www.ft86club.com/files/BRZwiringi.pdf it is plug HD2. doing a search on that pdf shows that it has audio amplifier signals traveling through there, so my suspicions may be correct. I have deeper bass now, and the audio doesnt sound like it is coming through a tin can. It may be worth checking your own connections if you have lousy stock audio.