sixth wire
so, inside the switchbox, what color is the feeding wire for the switches? black?
does this extra wire have voltage on it? is it possible it is the "always on" circuit? it was on my boat.
although mine may not be standard, either because it was so early int eh lifecycle and there may have been simplifications/design changes. also, i know for a fact that there was at least some modifications to the system because the battery was wired to the aft lasserette. but the red wire that feeds the switchbox to be distributed seemed original, based on how old the tape looked and how it was taped into the bundle with the other 5 colored wires. so i suspect that the modifications were elsewhere,but i cannot confirm that.
still check for voltage on the red one. and for giggles you could check for continuity to ground. since it seems like your black is the feeder (and should be red) maybe the red is the gound. who knows. the only way is to hand-over-hand it from one end to the next to find where it goes/comes from.
this is where you will want to hire some 8 year old gymnasts, to contort themselves into the cockpit lockers, under the cockpit sole, etc where no grown man is intended to fit. i felt like jackhammers had gone nuts on my lower back for a week.
-km
aka, "sell out"
S/V Beyond the Sea
C&C 35 mkIII