So, I have the same problem from leaking somewhere in the rear. Out of warranty. There is a lot of water inside the 3rd brake light shell. Where are you guys applying the sealant. Are you taking the light off and resealing it, or just keeping it in place and placing sealant around it?
I removed the light.
For my situation:
I have a vertical crack over the driver side of the square sliding window on the inside of the truck. This area is where I noticed water entering the truck. I also noticed this crack on top when I removed the third light on the outside. There is a gap between the bottom of the light and the top of the window frame on the outside of the truck. This gap runs along the back window down to the sides. I sealed with a urethane sealant from end to end just above the sliding window to start. Currently I am in my waiting period to let the sealant cure. There is a YouTube clip of someone doing this on a 2019 and it worked for him. He used a silicone product. I covered this whole area with sealant where the crack existed along with the rest of the area afore-mentioned. I used masking tape on the top of the glass to help minimize any mess.
My research resulted in possible sources when water is coming from the top.
1. Shark fin antenna. Gasket and/or sealing is not properly working. I found a YouTube clip that someone did to address.
2. Brake light itself. The seal is poor, the gasket is not lining up properly, or the gasket material is not durable enough. YouTube has someone addressing this.
3. Seal around entire back window is poor or the frame is cracked somewhere. Someone on YouTube did a seal job around the entire window to address this but don’t know if it worked out for them.