I just had the anchor light replaced on my 320. I had been paranoid that it was a bad connection between the mast plug and the deck plug and that I would be faced with the same problem you have. (In Florida, we don’t haul out except for bottom jobs, and it is very, very expensive to unstep the mast.)
Anyhow, it turned out it was just the fixture, and all is well. But the guy I hired to climb the mast said there was a fitting you can buy that allows you cut an access hole without weakening the mast. I assume this is not a big secret and that you could find it through some knowledgeable source or maybe even Catalina.
A funny story. There is a Catalina 28 down the dock from me, and he worked on it to fix an anchor light problem. He drilled a hole in the mast to install the fitting, and water shot out several feet. It turned out there was about three feet of water in the bottom of the mast.
Sonas, 1998 Catalina 320
Tierra Verde, Fl.
Sent from Mail for Windows 10
From: Marko Jamnik
Sent: Sunday, March 14, 2021 7:38 PM
Subject: [C320-list] Mast deck wiring entry panel