Hello Brandon B.
The question should not be - how to stop it from leaking, but why does it leak.
If you make friends with the people who works at your local Chevrolet / GMC dealership, you can usually go to their service dept and ask the technicians - what causes that problem to occure and what do I have to do to fix it.
As I spend most of my time explaining, the cure is not always the cause.
I will give you a very good example here. When discussing problems with a Chevy Truck, I use my brother Toms '02 Avalanche as a example.
The reason why I use that vehicle as a example is because everything that could go wrong with a vehicle - went wrong on that truck.
By my own admission - that was one truck that should have been recalled and scrapped by the PA Lemon Law. It was worse then a 1960 - 70's model Harley Davidson - AMF, where you had to have someone follow you down the road with a bushel basket to pick up the pieces that fell off while you drove it.
One of the problems we had was a wet carpet on the drivers side front in the wintertime. Living in Pennsylvania, your first thought might be that you came into the vehicle with your boots all covered with snow and that as the heater melted the snow the carpet got wet. Then as the interior got warm, the water started to evaporate and the windows got fogged up.
What you don't realize is that it was built in Mexico by Mexican's that worked for $17 a day and a bowl of Co Co puffs, and did not speak english and was damn glad to get $17 a day and a bowl of Co Co Puffs.
Some places, they put too much seam sealer - such as back in the C Pillar and the water would accumulate and would run out of the C Pillar when you hit the brakes and would run under the door sill plate and would stop under the carpet on the drivers side. Or - where they did not put enough seam sealer, such as above the parking brake cable, the water just ran into the cab when it rained or snowed and the floor got wet.
Either way, it was just a real crappy design, and the quality control was not there and so the problems just kept coming up. The Chevrolet garage knew that there was a problem with some models, but that GM was broke and they did not want to pay to fix every model with the problem so they just kept their mouths shut and declaired bankruptcy and walked away from all the junk they made. At the same time, I just got rid of that 02 and the rocker panels were already rusted out behind the passenger side doors. That is pityful. Especially when you consider - that I washed that vehicle several times a week and took better care of that vehcle then most people do of their own house.
My guess is that if you take off some of the plastic coverings that goes over the back panels around the rear windows of the vehicle, you will find the leak. I read several articles on that subject for your model of vehicle. Maybe some kind hearted individual in the garage will go as far as to tell you where to look.
I doubt if it has anything to do physically with the door seals, because you would have a whistle if they leaked and not just water on the floor.
