I combatted this by removing the third brake light and cleaning the areas with a microfiber and alcohol.
I replaced the existing third bake light gasket with a new OEM one.
I then laid down a small bead of RTV weatherproof sealant on the body side and let it set/cure properly.
I then installed the third brake light assembly (new OEM gasket mates with the fresh bead on the body side of the truck to form a tight seal that allows the third brake light to be removed at a later date with zero issues) and screwed it in place evenly.
Zero issues afterwards!
Whilst doing this, I also installed two new LED super bright white cab lights and an even brighter red LED brake light bulb, too (all CanBus/Resistor ready so I don't get any dash messages...lol).
The light output is amazing, and Stevie Wonder could see my brake light now as well
(there are some dumb ****'s out there, and making oneself more visible, especially when braking is paramount, IMPO).