Following on from other posts, I think I may have found part of the wiring conundrum for the reversing cameras.
When the original radio part of the sat-nav is removed, there is a second bit of wiring harness which becomes redundant, and which when properly connected goes into an extra socket on the back of the radio (in my case Alpine).
As it is not used for the proper radio function, it will possibly have been left behind and not cut out.
The plug has about 8 wires into it.
As I changed my radio myself, I dismantled the old radio to see what these other connections were, and discovered legends printed on the circuit board for each.
It does have a red wire (?switched 12v) and black (ground/-ve) with others, which may be the inputs for the video sockets in the side panel, as the video is controlled by the radio. The legends include R+, R-, L+, L-, ACC, Gnd(x2), Batt. Just got to sort out which pin is which.
There are several web-site for Honda groups which overall seem to agree that the red wire could be the positive lead to the camera, and of course the black one for the earth/ground.
There do not seem to be any video signal type cables in the loom, so perhaps those go straight to the sat/nav head.
When I can screw up my courage (I don't want to scramble the electrics!) I will try to check whether the red wire is in fact the switched live (hopefully NOT live if not connected via the radio), and then try a temporary hook-up.
More to come eventually.