I see what you mean, but my NA third brake light is pretty new and when I fitted it, I checked carefully the foam was uniformly applied. I should have told I also have JDM spoiler, but I fitted a rubber seal under each side. Wiper rubber grommet is sealed correctly. I suspect the glass corners (the ones close to the spoiler), the water is prone to stagnate there (bad design btw ! Some people have rusty corners, no ?) and I think some water passes b/w the upper rubber seal and the glass. Here is where the rain water drains out when the lid is opened up: