Roof leaking first noticed last year while camping on my 2015 Denali 2500HD, with a sunroof. Dripping from overhead and down by emergency brake whenever I washed the truck or it rained. I returned from Mesa back to Canada mid-April with a taped up sunroof to eliminate THAT leak while down there. To fix luckily I have a "Jimmy" who loves to solve car issues. We took off the side pillars, easy enough with a few screws, dropped the liner and found the drivers side hose not connected to the drain, front corner of sunroof. No wonder it leaked. Checked the other 3 corners and they were all secure, but we twist wired the connections so they don't fall off due to evil spirits. As others have said make sure they drain properly outside by pouring a little water in each corner before sealing up. Poured the hose to the roof for a few minutes, no more leak! This check should be done first before looking at the fin or taillight.
Now, second leak, passenger side floorboard rug was always wet, thought the hose connection was the issue, but while hosing the truck down we noticed there was drips coming in right-side kick panel in the corner. More swearing. Pulled the door side plate jam off to get some space in between the rugs to dry, was soaked. After Jimmy did some Jimmying, we pulled the wipers and the windshield cowling off, noticed there is an 4"x 8" access panel on the firewall metal ledge, passenger side. This was not sealed good, water came in around it, spilled into the cabin air filter inlet which was under the vent by a few inches, enough for water to get in, drip in truck, soak rug, etc. Pulled the plate, cleaned and caulked, plus put some of that flexseal super tape that the guy uses on the screen door to make a boat commercial. Put the hose to it, no more leaks. Sorry for the long explanation and forgot to take pics of this ordeal, but pretty straight forward if any leaks occur in your trucks. Pic below is the area where the access panel is below the cowling.
