You can do the test Mark said with the door open as well, just have someone stand on the mirror-side of the door and pour a glass of water on the window while you look at the bottom of the door on the inside. Water is allowed to go down inside the door from the bottom of the window, the question is where it goes from there. If the water deflector is leaking, you may see water come out between the door shell and the plastic door panel at the bottom. I've even done this with a hose since there'* no risk of it going inside the car. Normally it should come out the holes in the bottom of the steel door shell, which would drain outside instead of onto your inside threshold plate and then the carpet as you sketched.

update.

im now convined that the suggestion mark made was the correct conclusion. i went thru the car wash today (completely forgetting about the leak till id already paid for it, then figured id pay close attention), and i noticed that there was NO leakage during the car wash itself. it wasnt until i pulled out of the wash and into a parking space that it leaked. and it was almost all at once, too. that says to me that my thought about the door seals was completely incorrect, as according to that theory it wouldve happened when the water was being sprayed up at the car. so now i go to look for instructions on how to get off the door panel without damaging anything. this makes me sad. i so dont have time to do this any time soon.

http://www.bonnevilleclub.com/forum/...+panel+removal

Had the link saved in my bookmarks.