hatch leak source

I have been having a leak of some kind in the hatch of my 07 4Runner that is getting worse. After a heavy downpour, when I open the hatch, maybe a half pint of water will fall from the recess at the top of the hatch door to the floor. This started as a few drops several months ago. I don't see any obvious problems with the gasket, and the water doesn't fall until the hatch is open far enough for the water to pour past it. Are there drain channels of some kind that are supposed to send that water elsewhere that may be clogged? A few drops didn't concern me - I have the floor completely covered with a rubber mat and the truck has a few rough spots after 240k of fairly hard use. But the amount I'm getting now is going to lead to mold and mildew issues, and I don't need that. I'm also concerned about not being able to open the hatch this winter if we have a hard freeze following a downpour. Anyone else have experience with this issue and possible solutions? Other thoughts? Thanks.