"My guess is that the mast wires to the combo unit are the problem
Have they become detached or corroded?"
Yes, I've had what I think was the very identical problem a few years ago. My steaming light would not light up despite that,
1. I had twelve volts measured at the lamp fitting, and
2. I had a working light bulb when tested at deck level.
You are right to enquire whether the wires had become corroded, as on my B36(2002) the wiring originally fitted by Bavaria was ordinary copper wire, not tinned copper wire, and the copper wires had corrosion on them throughout the length of wire used from the switch board through to the light fitting, and while some of the volts were able to get through, the current or amperes were not able to get through in sufficient quantity to cause the filament within the bulb to glow sufficiently brightly enough to light up the foredeck. I changed the wires for tinned copper versions, and have not looked back since.