If the trunk door seal is good money then I'd look into adjusting the actual trunk door...you could possibly adjust the locking latch lower for a tighter seal but don't overkill it. The trunk door could be misaligned or it could just be the seal. One first things I replaced on my second 5thgen was the trunk seal, I ordered it online from Honda. That leak will ruin your interior with a moldy smell.Unlike the other threads, I believe this is has a less common cause, of which I'm yet to determine.
I changed the taillight gaskets a couple of months ago, as I was getting water coming in through the rear left, which was collecting around the spare, near the drain plugs.
Since then, I noticed a small gap between the metal of the boot just above the rear left, which due to rust, had moved apart slightly, letting water in (see below picture). I sealed this using some sealant, and again, it's been dry since.
View attachment 32184
Today, I had a look, after some heavy rain, and the wooden spare cover was wet, again on the left. I pulled the liner away, and found a pool of water had collected near the vent (I've since dried it out in the below picture).
View attachment 32185
I've checked the drainage from the sun roof by pouring some water over it, and I can see water coming through the drain hose and onto the floor perfectly fine (I had the sunroof shut, so only a few drops were actually coming through the drain hose. I did pull it out to inspect it, and it is not blocked.
I also checked the drain just under the fuel filler cap, which again drains without water ingress into the boot.
The surrounding body panels showed no black streaks of dirt carried by water ingress, similar to that in the first image.
The only evidence I can see is that the yellow looking seal towards the left of the photo was damp, and did have some black dirt, but the entire panel was covered in condensation, so it's not a confirmed indicator. I could not see any spots where water droplets were forming, nor where the point of ingress could be, unless it is that joint itself.