sounds like you may have to cut part of the ceiling out to expose the problem, if you dont have an acess panel that gives you room to work on it from the front of the unit/other side of the wall. i`d suggest putting a decorative access panel on the underside when you get done. good luck. p.s. have seen `drip pans` put in after the fact, to catch any future minor leakage and allow it to evaporate w/o doing more harm below. i don`t like this because of the moisture it allows to collect between the joists. in an older/not tignt house, this souldn`t be a problem.