I can’t remember the layout of a 321, but in our (once Dr Vic’s!) N33 I put the changeover switch in the little floor-level cupboard under the left-hand seat in the wheelhouse; where the Eberspächer heater and incoming mains switch sit. (I think the autopilot’s in there, too.)
Eight-year-old battery? If it’s been run flat in those 8 years it’s unlikely to work at peak performance now. And being constantly charged? ..You do check the liquid level inside the battery from time to time, don’t you? Even at constant trickle charging, some of the water’s bound to disappear after a while, and that’ll reduce the batt’s capability.
(I’m off to our boat now, to check the batts, etc, so may not be able to reply again till Friday..)