The bty has an integral charger circuit and has red, black and white wires going to the GPS mobo. Thinking that the bty was stuffed, I was surprised to find it was reading 3.9V. Not the bty then. The white wire comes from the charger cirduitry via an external small capacitor so it must carry an AC signal.
At this point I am stumped. Would very much like to know how these power systems work but it is not looking good at this stage for my GPS unit. Pity, as it otherwise works fine.