It might be unrelated but I had this last winter and it took me months to solve, I had a double issue, the neoprene seal around the rear high level brake light had gone, I had fitted the clear lens version from a later model and for some reason it never really sealed properly so I went back to my original red lens version with an aftermarket neoprene seal but it still didn’t seal, I managed to source a genuine LR seal and it’s important you get absolutely every last morsel of the previous seal off before applying the new one, but this only halved the amount of water I had coming in so I still had an issue, if you remove the plastic fascia that covers the top of the tailgate on the inside it exposes the back of the brake light, I could see beads of water forming along the window line and workshop blue roll confirmed water was getting in through the window seal, I got hold of some window fitters sealant which goes in a simple caulk gun and put a generous bead along the window line from the inside, it doesn’t matter how messy you are really because the plastic fascia covers it, the problem stopped over night and touch wood a year later it’s still dry, this might not be the same problem you’re having but it’s worth taking off that top fascia on the inside and checking, I remember at the time I posted this problem and lots of people said there’s a rubber grommet in the boot well that sometimes comes displaced letting in water so that might be another avenue that wasn’t the problem in my case though