It seemed to me like they were located on the engineering hull.
Engineering hull, near the shuttlebay.
Actually, I got the impression that the torpedoes never left the shuttlebay and their launch tubes were retrofitted to a location relatively close to the warp core (hence Scotty being so pissed about them; he later bitches to Keenser that "it's not just
the retrofits..."). I also figured those tubes were probably converted from existing escape pod tubes or something of that nature; existing features re-purposed for this mission.