I've started! First was to locate the USB module. It's under the right hand boot trim cover, so floor out, rear battery cover off, slam panel trim (4 torx screws) off, few plastic trim buttons off and you can move the trim piece aside, no need to disconnect the boot light.
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USB module is the silver rectangle in the middle of the picture.
Unplug the lower right connector and you need to release the inner fibre plug from the outer plug, there's a small tang.
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Find the 2 supplied fibre loop adapters, you need to remove the fibre loop from one, discard the loop but retain the outer housing.
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Fit the housing onto the MOST connector in the car and plug the loop adapter onto it.
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Plug the electrical connector (now minus the MOST fibres) back into the USB module and tuck the MOST wires neatly somewhere.
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Put all the boot trims back together. Next stage is the infotainment. But I need dinner first.