[aleekat]

Since you have your panels off. Get some cheap baby powder/talc. Shake it on the vertical surfaces. Then I would run the garden hose, starting at the lowest point or where you suspect the leak is coming from. The water will leave a "snail trail" in the talc. It will help you narrow down how it is getting in. Is this by chance a extended cab? I tried to see pics of the silverado. My Ram has a piece of rubber molding on the roof above the rear passenger windows. Under that rubber trim is a body weld. Thats where mine starting leaking, it would run along that weld, works it way down the back wall and enter the back area. Run down the edge until it got to the sill and move up front. I had this leak on both sides and took me forever to find it the first time. Got some exterior silicon house chalk, pried the molding up and sealed the entire weld then put the rubber trim back down in a bed of the silicon.Good luck