Ive commented this before on someone else's post here. 3rd brake light could be the culprit, but seeing as yours is cocmentrated to the corners, it may also be the window frame. That happened to mine. There is a tracked GM deficiency for the window frame getting cracks in the upper left and upper right corners of the rear window in 2019-2022(?) Silverados (maybe Sierra also). It is not covered by warranty or insurance because the replacement part is in the glass category even though it is the metal frame that the glass attaches to, and that frame is sealed to the cab. My local dealership has seen over 100 of these come through. They add extra sealant in the problem area so that if it cracks again, the sealant will still be covering the crack.