It looks like I'm going to have to do some serious wire back tracking to clarify the situation!
To further describe the setup in the cockpit:
1) The wires coming through the bottom of the cockpit table include the Raymarine MFD ST cables
and other connectors, and a separate 12V junction leading to the cockpit table light switch and the 12 V "cigarette lighter" socket under the table.
As far as I can tell, the Raymarine MFD cables go directly to the main DC panel and are controlled by the "plotter" switch on the main panel. They do not seem to have interference
with other devices - to the best of my knowledge
However, the DC socket's wiring has evidently been tinkered with by PO, because it only gets powered up when the "Instrument" (NOT the "Plotter" switch) switch is on, so there must clearly be an as yet unidentified connection there that needs to be verified, I will do so when I'm next on the boat.
As regards the NAV lights, there is no connection, as far as I can see, between the cockpit table wires and the Nav lights wire, these obviously are in a different location. However, the reason I had mentioned them in the first place was that I had found a section of loose cabling underneath the cockpit table, with a partial identification as belonging to the the NAV circuit, but this is not definite.
On the other hand, since the only thing that is different between the day and the night are the NAV lights, my electronic engineer
friend seized on that confusing wiring situation as the main culprit...
To Dave's point, yes, it does get pretty hot during the day, and at night the humidity is often bad enough that the deck
and cockpit are wet from the condensate.
I guess I really have no choice but to backtrack all the cables - and replace the Raymarine ones which are fragile anyway-.
If the cause was humidity, then replacing the cables with hermetically sealed ones should hopefully fix the problem.
If the cause was due to incorrect wiring (ie, the same DC cable possibly shared by more than one device) as a learning
opportunity, can someone offer a theory as to why this might cause the day/night cycling? ie, why would it work OK during the day and why not at night?