Well I've verified the 2 important wires:
Black with blue trace = ignition + 12v
Brown = Ground
And tested it works by jury-rigging the lead to a 12V DC wall-wart and covering the sensors appropriately.
Any thoughts from anyone on how to get safely into the overhead lighting/sunroof control area and what to look for?
I'm not bothered about the reverse feed at all; I didn't connect that on the unit I fitted to my Focus and don't ever miss it.
(tend to use windows, side mirrors, eyes and brain, assisted by the rear parking sensors
)