On my boat, there were three power feeds and one neutral feed (although I seem to recall multiple neutral wires connected together?) broken out of the main harness and fed into a round 4-pin connector that attached at the base of the mast. I believe the three power leads went to the panel switches marked Spreader, Masthead, and Anchor. This is interesting because I've never seen anything resembling an anchor light on my boat; I just have a foredeck and a steaming light.
I did replace that connector on my boat because I thought it was corroded/not making contact. I'm not sure if this was really a problem or not, since there was severe corrosion on the bulb contact for the steaming light which prevented it from working. I went up the mast with some sandpaper and fixed it. If that fixture is what I think it is, the contact itself is solid, un-tinned copper. Your wiring might be fine; it is entirely possible the problem is up in the fixture.
