Appreciate all the replies! They inspired me to try to fix it right.
I first detached the ribbed cable shroud where it meets the smooth shroud that disappears into the body (at yellow arrow in photo below), this was just taped on.
I was able to expose about an inch of wiring. I pulled on the appropriate wire to see what happened... and it popped out.
I then used the broken wire length as a guide and carefully cut away a section of the ribbed shroud with a razor blade and a wire cutter.
As others suspected, I had other problems waiting to happen -- possible shorts of hot to ground wires.
I spliced the broken wire with a crimp-on heat-shrink butt connector, put some electrical tape of the broken insulation on the others (the wires seemed to still be ok for the moment).
Wrapped it all with some rubber splicing tape, pushed the shroud over the rubber, then another layer of splicing tape on top.
Sitting inside the trunk and closing the trunk lid quickly reveals why this is a problem spot.
I doubt my fix will last another 19 years as the bent wires are probably brittle and the butt splice has also made things more stiff, but I guess I'll know where to look in the future!
A more permanent fix might be to splice all the wires, adding a loop in such a way so the loop gets smaller/bigger when opening/closing the trunk rather than forcing wires to bend.
I hope the photos help someone else in the future, thanks again for all the replies.