It turns out the last cable that I could not trace could not be traced because it didn’t go anywhere. Anyone with the older Raymarine/Autohelm gear can tell you that instrument displays like the VMG have a triangle of connections – two are for the SeaTalk1 bus (in and out), and the top third one goes to a transducer of some sort. In this case, I hadn’t twigged on that fact, and I had to take the helm console apart again to realise that the cable I couldn’t find was an open SeaTalk bus cable that tucked under one of the other black, round cables, and I had confused the two. Oh well, now I know.
