So I assume after it rains your headliner is not wet in that back corner? I had water in the same area and it was my sunroof that was the problem. The water drips down along the c-pillar, goes under the back seat and on the floor. It wouldn't hurt to check the sunroof drain hoses if you haven't done so yet.
As stated, it could be the door weatherstripping, or the plastic channels too. But when I fixed my leak, I resealed everything that it could have possibly been, even if I thought it wasn't the cause.
Note: please put 92 ssei in your signature just so you don't have to remind us when you have a problem in the future
Go to profile at the top of the page, then edit your signature.