That's the idea of putting a bit of water in there, it may draiin a little, marking your spot, you shouldn't have to take any metal out of the way though, look for either a hole, or i know on some vehicles, you may actually find a drain hose under there

 

I'll bet money if you remove the door trim where it runs along the pillar you will find crud,I had the same passenger side leak, I removed the seal and cleaned it good and it hasn't leaked for two years now....

 

Schlep, do you mean the front piillar where it angles down forward from the top of the truck? How did you clean? With it on the truck? Yes, it may have oak tree "residue" (crud) on/in the crevices. Hmmmmmm.

 

Mine had crud/dirt built up where the rubber seal goes over the metal ridge,but it was cruddy all over,yes down the passenger side pillar/slope down the front,I took the entire rubber seal off of the truck and cleaned it with a hose and a wash rag/towel,be sure to clean the metal ridge as well,just mind where the joint is,at the bottom....

 

More info!!
Okay, this morning I totally cleaned all the door seals on both sides. Yeah, they were kinda crudded up......dunno if enough to cause the leaks.

ALSO, I removed the kick panel in front of the passenger's feet to see what was there and found the sound proofing on the back of the panel was very wet on the bottom right-hand corner (4" x 4" looking forward). I cannot get my head under there to see what might be dripping (or soaking up from) at that spot. I'm not sure water stopped up in the door seal could get to this spot.
Does this shed any light on the problem for anyone?

 

I don't know,cleaning the door seal fixed my leak...

 

I understand and am hoping the door seal(s) fixed it. I sure FEEL better now that they are clean!! Fingers are crossed!

 

I have the same problem. 99% sure mine is the door seal. I know my bottom seal is shot, noticed some muddy water on the kicker plate.

 

Finally able to say, that I believe cleaning out the door seals was my problem. It's an easy fix.......just pull the seal away, pry it open and clean it out with a somewhat stiff brush. I used an old toothbrush. Then just push the seal back on. Go all the way around doing this. REALLY EASY, and it sure seems to have stopped the leaking onto the floor. The truck lives under an oak tree, so that's what was clogging the seal up and not allowing it to channel the water correctly.
Sorry it took so long to confirm the fix but we haven't gotten much rain in this part of Florida, and I wanted several good hard rains for the test.
Thanks again for the board members who responded and offered suggestions to my issue!