I believe the splices are probably going to be somewhere around the RHR light, either in the 'C' Pillar, or above the headlining, as that's where the loom splits off to go to the RHR light, and then forwards and around to the sunroof etc. (from memory) with the other side of the loom running along the back to the top of the tailgate.
If you pull the tailgate seal away at the top, you should be able to get into the space above the headlining card and get to the connectors. It would probably be worth checking for voltage there, as it will tell you if the issue is in the vehicle side of the loom (back as far as the splice if the other lamps are working inside the vehicle), or if there is voltage there, then the issue is in the tailgate section of the loom.
I'd still be looking carefully at the last few inches of wiring to the boot light itself. If that's where the spark was, then to me, the problem must be around there. As I mentioned before, my boot light just stopped working (I swapped it for a couple of LED panels ages ago) one day. When I got around to looking at it, one of the wires had corroded inside the insulation, and broken right through, so no voltage at the connectors. The other wire had corroded through at the connector, and pretty much fell apart in my hands.
'06 4.2 Supercharged Black/Cream "Jess" -- Bought April 2021 (NZ)
'01 4.6 Vogue Black/Lightstone "Snog" -- Bought Jan 2012
'99 4.6 HSE Cobar Blue/Lightstone -- Sold March 2009
'95 4.6 HSE Epsom Green/Tan -- Traded June 2008