This type of leak on this type of boat can sometimes be due to leaks in the hull-to-deck joint under the rubrail. Water runs off the deck during a rain, runs out the drain channels and onto the rubrail, then heads aft, getting underneath the rubrail in the process. There's a hole under the rubrail every six to eight inches that has a bolt through it to hold the deck to the hull, and it's not unusual for some of them to leak. We had a similar aft cabin leak (our boat's smaller, but built basically the same way as yours), and I cured it (it's not fixed, I've just treated the symptoms) by running a bead of caulk along the top of the rubrail from approximately the midship cleat to the stern. Now the water that runs off the deck continues along and over the rubrail without getting underneath it, and our leak is gone.
