I searched around this forum and found a few threads about fixing this problem but nothing was definitive enough for me. I also saw some misleading information posted about the size of the drain nipple so I just want to clear things up and make this post the resource. I didn't take pics of my procedure but there are some floating around. I'll take a few pics next time I get a chance.
First off the reason why water leaks back into the cab is because of the stupid design of the drain system dodge put on these trucks and other vehicles. They used a foam gasket to block the hole through the firewall which the drain tube goes through. It's literally just cheap foam weatherstripping which after a while deteriorates and either opens up or allows water to be absorbed through it and leak into the cab. The other thing that they neglected to do was make the tube either have a 90deg bend on the outside or to simply provide a 90deg rubber hose stock. The fact that it is straight out lets water drops follow the tube back and run down the firewall. After the cheap gasket gives out then it can easily run inside the cab. FAIL! Another reason some mention is the possibility for the drain to become clogged. I don't think that in most cases that would ever happen unless you are offroading a lot or mudding. For a street only truck this shouldn't ever happen.
I saw in places that the size of the fitting needed was 1/2" which isn't true for my truck which is a 2001 4x4 (not sure if there were differences out there between yrs/models). My truck has a 3/4" o.d. nipple which was very hard to find something to match up to since this is not a common automotive hose size on most applications. The standard plumbing fittings from the hardware store would not work for me, plus I wanted something rubber that would snugly slip on without having to use some type of glue to hold it on.
I tried the recommended hose from the dodge caravan, but that is also 1/2" and did not fit. I also called the dealer to try and look up the TSB for this fix and they couldn't find anything that gave a part number for a hose to use. They told me all it said was to seal it with RTV, which is near impossible to do.
What I found that worked perfect was a Dayco hose P/N A70647 which can be purchased at many autoparts stores for $6.50. It's a 3/4" i.d. hose with a 90deg bend and is 3.5" long which is more than enough to do the job. It slips on snug and holds good. You could also use a hose clamp or zip tie for good measure but I am trying it w/o for now to see how it holds. I will prob put a zip tie on it later. I would like to get some kind of heat shield for this as well and if I find something I will post it up since this is in close proximity to the exhaust.
As for procedure, the install is very easy, just a very tight place to get into. Also note that you will want to do this when the truck is completely cool, or risk getting burned on the exhaust manifold. You will need to remove the wheel liner which is plastic riveted in. You just need a flat head and pliers to break off the heads of the rivets and push the center pin back though the holes. I have a good clip to put these back on with w/o having to re-rivet them which is great since most people won't have access to that. I'll post that part number later. After removing the liner you have full access to the firewall where the nipple is located. Now some people say you need to remove the tin heat shield, but on my truck I didn't have to. Nothing was covering or blocking the nipple. You just have to reach in there and feel for it. It's all the way in next to the tranny bellhousing. It's the only thing protruding from the firewall and it's big enough to stick your finger in, and it should also be wet from condensation :jester: I just realized what that sounds like lol. All you need to do now is slip on the new hose and put the wheel liner back in using the new clips. :woot:
Clips are a GM/Ford item. GM #14093088 FORD #N802781S.
If you have more information about the different trucks with 1/2" vs 3/4" please post it up. Maybe just 4x4 models are larger for some reason, or could be 01+. I would like to know.