Hi TT
You should try it on Sonja with the same trick for a new sounder.
Had problems with an anderson plug recently & had another fitted by Battery World. I now know how to fit them properly.
Strip sheathing from cable & slide on heat shrink.
Coat wiring with solder & also completely fill with solder the terminal fittings
Hold the terminal fitting in a vice & heat with primus torch, then slide in wiring. Cool then crimp & finish with the heat shrink.
Its probably how everyone fits them only not having an electrical background I was impressed.
Im putting a plastic bag with duct tape on the fitting when fishing the Seaway now as I sometimes I take some wash over the bow.
Coming up to Somerset soon let us know what you have worked out. Denis