Looking carefully at those picture, the cable clip does NOT go thought to the outside of the car, just a void between two panels, and the water comes from the void.
This is the clue, there is a big long seam between these panels, going all the way around the lamp cluster and up to the boot aperture surround.
See this superb picture from the thread “Hooded Reaper’s NC1 Sport”, and follow that seam.
I am betting that rear panel on your car has been dinged or the wing has or something like it to stretch it somewhere, and those seams are no longer 100% sealed. Bear in mind the main drainage for the water comes out right above the centres of the lamp clusters, any potential leak points along the seam will be flooded.
A simple solution is to take off the light clusters and bumper and re-seal the seams with appropriate sealant, then repaint over it to make it look original. There are lots of on-line video about “Automotive seam sealer,” some are less gruesome and sales-centric than others.