A wet boot

[BarneyB]

Hey guys, happy new year to yall!

So its looking pretty soggy back there, filling up the wee cubby hole in the back left of the boot and running down into the spare wheel well. I've read about the rear light clusters getting gappy and letting some wet in so had a look at mine. No seals to speak of in there but there is a metal panel join (near the top of the cluster) that has been sealed up before which is now deteriorating.
Is this the likely ingress point or am i off target?
Thanks in advance
Barney