In my case the drain tube had fallen off. Once I put it back on it's been dry. It's easy, just remove the pillar after removing the two bolts holding the handle on and pull down the headliner. I cleaned the nipple and inside the tube and used a couple of drops of super glue to make sure it doesn't fall off again.
On a side note I was wondering where the blue stuff on the back of my headliner was coming from. It occurred to me it was windshield cleaner. I pulled the upper rear brake lens and sure enough the tube had fallen off there too.