Where I put mine
We have a 1991 30T as well. I decided the best place to install speakers was on the forward main cabin bulkhead, facing into the cabin. I bought a pair of Pioneer 6 1/2" 3 way speakers with polymer cones, so as not to be affected by long term exposure to high humidity. I like Pioneer auto speakers best, because they consistantly deliver better low frequency response than any other brands, and they usually are very efficient, which means more volume with less power. I installed one on each side of the forward cabin door, mounted as high as possible, but such that the rear of the speakers would still be inside the two lockers in the forward cabin. I also installed them as far apart as possibe to provide maximum stereo separation in the cabin. Installation in this manner allows the forward lockers to act as resonant chambers, similar to home speaker cabinet enclosures. This greatly enhances low frequency response. Since the speakers are aimed rearward, they can be heard reasonably well from the cockpit while under sail, or at the dock, as well as throughout the cabin. I cut the holes in the main buIkhead by drilling multiple 1/2" holes along a penciled outline of the hole to be cut out, and then using a sabre saw to cut between the drilled holes. You'll want to remove the cushions, and tape some plastic tarps against the cabin sides, and have a vacuum handy. The sawing makes a LOT of dust that seems to go everywhere. I installed a car stereo deck on the port side of the main cabin, just underneath the shelving, between the two cabinets, toward the aft cabinet. I made a wooden face plate just large enough to hide the rear of the stereo from general eyesight, with a cut out such that the stereo could be slid in and out easily. I rigidly mounted the face plate with (3) 2" angle brackets - (1) on each side of the plate to the bottom of the top shelf, and (1) on the aft bottom corner to the side of the aft port cabinet. I ran the power wires along the bottom of the top shelf (attached with nylon wire mounting loops) aft to the main power panel. The starboard speaker wires were attached to the back surface of the main bulkhead with nylon wire mounting straps, inside the forward cabin locker. They were routed to the port side, along the existing wires for the starboard front cabin light. The speaker wires were then routed along the bottom of the top shelf on the port side, aft to the stereo deck. This installation sounds great, the wire routing was easy, and everything is inside the cabin, protected from elements. The wooden faceplate makes the stereo deck look like it was factory installed. I hope this helps you out.