Your Bentley is accurate according to TIS. After X138 (the connector up in the boot lid) it is a single black/yellow wire all the way to the LCM. I think I would start by testing the black/yellow wire in the right side of the rear compartment (should be in one of the bundles running down the right side D pillar). If you have +12v there (or whatever voltage the LCM uses for the light) then your likely culprit is a break in the wire at the rubber boot thing where the wire would go from the body of the vehicle. Very common for those wires to break after years of opening and closing the boot lid.

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