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"I am adding an anchor light to the mast of my 1975 C&C25. There exists a masthead light and foredeck light,"
Is that masthead light a white all around light, as in an anchor light? Or do you mean it is a red/green nav light set?
If I remember correctly, there are some schemes using 3 wires for 3 lights, so you would not have to change the cable or connector. The trick is that they add two diodes to one of the hot wires going to the masthead and use a switch to reverse the polarity on that wire, so that when it is fed one way, it powers one light. Fed the reverse polarity, and courtesy of the diodes up top, it power the other light instead. (i.e. one way, the nav lights. The other polarity, the anchor light.)
Offhand I don't remember the details, only that there IS a fairly simple way to make that happen with a couple of diodes and switches.