Buckely

If the light/buzzer is coming on, you have a voltage across it. Obvious but true. Measure it, and measure the voltage each side wrt ground. If you have a MPPT solar controller moving to shade doesn't prove much. The controller can step up the voltage as well as reduce it, for maximum power transfer. The current will of course go down if the output power reduced. I suspect that one side of the lamp is being held high by the solar, the other side is going low enough when the alternator cannot main 14.2 v, due to high current, to cause the lamp, buzzer to come on.

Take into account how the cable will be routed when selecting the cable size. The IEE regs BS7671 require the cable capacity to be derated by (from memory), 50 % if 500mm or more pass through insulation. I know BS7671 doesn't apply to boats, but a fire ( or 'potential' thermal event as the legal department of a previous employer insisted we call them) is a fire!