There are two bowden cables. One goes from the release lever in the boot to a compartment in the side of the backrest (you can see the cover if you fold the backrest down) and the other cable goes from this compartment to the latch mechanism at the top of the backrest (the one you'd use to fold the backrest from the rear seat).
The two bowden cables join in this compartment with a small rectangular metal coupler, described in the manual as either a "connector element" "connector piece" or "connector element block".
Well I only went and dropped it and it disappeared somewhere in the guts of the car

It's the bit labelled 4 in the second diagram...