They had this "thing" back then to run the coax for the VHF and the 12v wires into a foam sleeve and tape the hell out of the ends, and every so many feet. Said foam was to keep the wires from slapping against the inside of the mast while at anchor and crew trying to sleep.

So if the coax is original ... probably you should pull the whole harness and do both at the same time. Coax is not so great after 20 years or so at VHF.

The resistance you may be getting is that wire being held up by the coax to the antenna. Of course if you don't have an antenna at the top of the mast ... another story.