Further progress. After removing the aluminium panel behind the rear wheel (10 screws) I found the two rear light plug/socket connectors. Previous research indicated the four way one carried the fog and reversing light supply. As expected the green/brown (switched +ve) and black (-ve) reverse light cables were not matched by connections in the plug part. Just two wires were present here feeding the fog light. After confirming 12 volts was present on the reversing light cables, when reverse gear selected with ignition on, they were cut close to the socket and extended to reach the rear light housing. Temporary removal of the leather check strap eased the new cable routing alongside that of the number plate light.
The rear light cover is secured with three M5 screws. They locate onto rivnuts on a supporting bracket. The upper facing one was found to be the ideal location to mount the reversing light pod. It was drilled out to leave a 9mm hole, being the tapping size for the fine thread M10 support stud. An M10 clearance hole would have left minimal metal on the 16mm wide bracket. After tapping, the M10 stud was fitted with washer and locknut below. The lamp pod was then assembled onto the hollow stud with shakeproof and rubber washers between, and connections made to the new cable.
The plastic lamp cover was then carefully cut to form a snug fit around the rubber mounting washer and seal against it when the two remaining cover screws were tightened up .
Finished result.
