[TKCR]

Waterleaks are hard to chase down. I work in an auto manufacturing plant (not GM), and finding & repairing waterleaks was a job I did for quite a few years. Where I work, we waterleak test & check every unit we build. Not all plants do this. They only check 1 unit out of every "so many". This is how leaks get out to the customer.
Can you give more of a back story on the truck? Have you had the truck since it was new? If so, was the leak happening then?
Does the leak only happen on really hard rains or any type of rainy day?
I know its not the best time of year to do this, maybe wait until next spring/summer. I would take the truck to a car wash, one where it can just sit while the water is running over the whole truck, at least the cab. Now comes the fun part. You need to try and lay down there on the floor with a flashlight, so you can see up and and possibly get and idea where the leak is running from. If you see the leak, try and trace it up to where it may start. You may see something and then you may not, but its worth a shot. This is the method we did, when the leak was coming from under the dash. We would put the car in the booth and lay on the floor the best we could. Then wait for the leak to happen and get an idea how far up the water was coming in at. You may have to remove some of the lower dash covers, to make it easier to see up in there. If your able to get an idea on where it maybe coming from, then you can take it home and start using the hose on specific areas.

Some other possible causes, it could be a grommet/blind plug missing. This is a common issue at the plant. On the assembly line, people forget to install their plugs. Check the door harness boot to the body, this is another common area. The boot isn't set completely, so water gets thru. I have personally had 2 Chevy trucks now, where the door boot has not been set correctly. A new, at that time, 2007 Chevy Avalanche. And then a used 2006 Chevy Colorado. The Colorado had been wreaked and a new door was put on the truck, the door boot was never set. This would be another back story question on your truck. Has there been any work done on the truck? Maybe something wasnt put back together correctly, causing a waterleak.

Don't try and do this on a day where you are on a time limit. Chasing a waterleak can be frustrating, so give yourself plenty of time, it can turn into a project. Keep your patience in check, once you lose that, you might as well stop. I know I get like that, once I get mad or frustrated, I start missing things because I'm too busy cussing at everything, lol.
Good Luck and keep us posted.