Any ideas on how water is getting into Corsa (2001) boot?

I have seen other answers about water coming in through seals, but I don't that that is it. It seems to be coming from the back corner of the boot floor, seeping in through the weld? I am loathe to get a welder in if there is a simpler solution (other than put underseal over it to stop and hide it), and I might be looking in the wrong place.