Was your truck sitting a long time before you purchased it? Mine was sitting for several years under a tree and collected a lot of junk in the drip rail which caused the actual roof to rust, oddly enough the drip rails were solid, probably from the seam sealer.
The water was running down through the frame and out the seam under the dash. It pooled exactly where you say yours is. I thought it was my windsheild seal, but I wasn't that lucky.
I couldn't see any holes in the roof but I knew it had to be coming from the top. I pulled all of the seam sealer out of the drip rail and found quite a few rust holes under it.
I took the truck to a good body man and he cut the rusted areas out, welded new metal in, bondoed it, added new seam sealer to the drip rail and primed the roof for me for $300. It looks like new and does not leak.
I hope this is not the case for you but if you can't find the leak, I would suggest taking this approach or take it to someone that knows. Hope that helps.
