This black conduit is that bestatchess is talking about is acutually a grommet that redirects the water to a specified area. The reason that you guys are not seeing it when you are doing your "tube extension" modification is because it fell off during transit. First of all, all of the trucks are delivered with it, but they fit like crap. I noticed this when I went to locate my problem originally. I had to push the piece out from inside of the truck. It fell out with almost no resistance. When I went to put it back in place, it is really loose inside of the hole. It really is a good idea to have this grommet in place because of the purpose it serves.
Also, Bestatchess, this part will not keep water from entering the cab at all. When it is installed, it is not form fitting around the nipple that exits the truck, so the water can easily stip drip back into the cab if the nipple is not properly angled. That is exactly what originally happened on mine. Now that it is fixed, I had them reinstall the grommet with a small dab of silicone.