You checkbthe shark fin up top on roof? Gaskets split and water leaks in there... especially on the back side..
I have not yet checked the shark fin. I will check that this weekend.
I am having the same problem but not as bad. My pad at the firewall is dry, just getting water on the floor. I was thinking of taking the A-pillar cover off and spraying the vehicle down with water and watching that area.
I'm getting water from the firewall as well. It's a constant leak.
Has your truck ever had the windshield replaced? Look for the GM logo on the passenger side. I had the same issue on the passenger side floorboard, pulled everything out and i pulled the headliner down, you could see daylight through the 1/16th inch gap probably 2 inches long they didn't seal very well.
Also the headliner has a hard coating on the back side, so if water gets on the headliner from the roof, it may not show through.
My windshield has never been replaced. It's the original GM windshield. It has the GM logo and says AS-1 on it as well.
I was wondering that about the headliner. Every single post and video I've seen say that it eventually got stained. I only saw one video on YouTube where the guy said that it didn't show.