probably upper back panel, roof, hinge bolt seams or wiring harness. They leak up top and follow the sheet metal of the vehicle to the lowest point in the spare tire well. It could also be a seam up underneath liftgate weatherstrip. I have my own caulking i use to fill the channel of the weatherstrip and place it back on the liftgate to stop the water from going up and over it.I think in another thread someone mentioned there was a gap in the weatherstripping or something to that effect. Search for trunk and water, or something like that. I'll try finding it later.