Most likely, the rear drain homes are plugged or have come off. The other possibility is that when you plugged the front ones, the water now hase to all go to the rear, and might be overwhelming the rear drains and overflowing onto the headliner.
Either way, to check the drain hoses to see if they are connected, you'd have to pull the headlinder down some. This would require removing the rear c-pillar covers, removing the rear grab handles, and then pulling the headliner down some in the rear.
Might be able to get some pictures later.