As Nick says, this wire is part of your antenna. It radiates just like your insulated section of backstay. Just make sure the part inside the lazarette is away from other wiring
and bonded fittings and you should be fine. You should see the way the idiots that wired the new fleet of sloops at the Naval Academy did it. They installed the tuner on the port forward bulkhead of the lazarette and put the thru-deck fitting on the starboard side. They had some 15 feet of gto15 running inside the wire loom with the radar
cable! These boats also have the split backstay like yours. After delivery
of the first 7 boats, I finally got them to have the wire exit the port side but they are still running it in the wire loom. Iv'e had to re-do all of them (12 boats so far). The gto15 is run through 3/4"O.D. clear water
hose which is tie-wrapped to the stay. They initially had no protection from water at the top so I just sealed them with rescue
tape rather than put in a loop of hose. You should also see the way they attached the wire to the backstay, unbelievable!!! The list of poor installation
practice on these boats goes on and on...lowest bidder??? Your tax dollars at work.