A wiring fault?

[Papawads]

Everything now works but...
After a radar installation on the mast and other mast-electric tinkering, I had the oddest fault. The four mast-light switches at the captains table each showed 12v at the 4 mast base connectors, when not connected. But when I connected ANY ONE if them to just one of the 4 curcuits in the mast, ALL FOUR mast cables showed 12V, even through 3 in theory have no power! With all four connected, no mast lights worked. So as this point I'm thinking "we have a short", right?
Then I realised, the negative from the mast, wasn't connected to the negative on the boat. When I made this single connection, everything works fine. Interestingly, back at the captains table, when I switch on Any of the four light, the little red lights by all four switches come of for a fraction of a second, barely a blink, and only the correct red light stays on fine.

Anyone got any ideas about my non-fault. BTW the radar is a completely different curcuit.

Thanks guys 👦