Ogre, where do you run the leads to for charging? Do you have a photo of that?
This image is called an SAE connector. If you are in Europe, they may be hard to find but here in the States, they are very common.
This connector is at the end of the terminals that are connected to the battery. I plug in what ever accessories I want, using the mating SAE plug.
NOTE: The hot lead of the SAE connector, that is attached to the power source side, must always be the recessed part!!!
The protruding lead is always negative, so if it contacts anything, NO WORRIES.
REGARDING GPS...
Each of my bikes has a RAM ball available to attach a GPS mount. They also have an SAE connector. This way I can switch units easily between bikes and they are easy to remove, so that nothing is left on the bike.
To do this, you must mount the power converter, of the GPS mount, on the MOUNT...NOT on the BIKE. This way all you need is access to a fused 12 volt (which is supplied by the SAE plug).