The lights in the head have stopped working. The multimeter shows 0.9v at the ends of the main cable carrying current up there. That means there must be a break in the cable, and probably some corrosion. Since it's one of the few remaining original cables that I still have from when I bought the boat, probably a whole lot of corrosion.
On Saturday I'm off to Wollongong but I guess that on Sunday I'll be pulling and replacing cables. Joy.