Trunk leak in 2007 Grand Prix

I have a pervasive trunk leak on my 2007 Pontiac Grand Prix. The leak appears to be coming from somewhere in the "ceiling" area of the back left part of the trunk (using the center of the left tail light as a reference point, go ~6 inches towards the front of the car and the leak is somewhere on this upper left inner face of the trunk). The leak manifests on a surface in the upper part of the left turnk which faces away from you when you stand at the back of the car looking in the trunk. Its very difficult to even see this area, unless you are lying in the trunk with your head pressed against the left trunk wall looking upwards, and use a bright flashlight. On top of that, the leak is very difficult to reproduce (it may take ~20 minutes of moving the garden hose up the side of the trunk very slowly until water starts to pool down this inner trunk surface). I suspect the leaked water comes in through a trunk hinge (as the trunk hinge is located in "aqueduct" area where water flows around the trunk), fills up in some space between trunk metal sheets, and when it fills up enough, it leaks out a microscopic weak spot in this hard-to-access area of the trunk (this "fill-up" theory is all I can come up with to explain the delay in reproducing the trunk leak). I've had the used car dealer that I bought it from try to repair it, when they couldn't they sent it to two car repair specialists in my area (glass and molding specialists). Frustrated with having my car in the shop all the time, I have tried several times to apply epoxies, caulks, and all sorts of sealants to any area I thought may be in the leak pathway (in the "aqueduct" area alongside the trunk rim, as well as on the surface in the trunk where I think the water comes out ... yet it still finds a way through). The result of all these efforts were a reduction of the leak flow rate to about 1/4 of what it originally was when I got the car. I usually place a tarp over the car when not in use for a while, but I now have a square plastic bin that I have in the trunk below this area and will empty it out if it rains without the tarp.

My mileage (~35k) is under the basic car warranty level (36k miles), but I have not yet brought this to a GM dealership (didn't really consider this option, as all the other professionals that worked on my car couldn't fix it, and I don't like having my car in the shop for days only to have it come out leaking again, but now I am reconsidering).

Any "quick fix" ideas? Would it help to take this to a Pontiac dealership, see if its a warranty issue?