There are two existing 3 amp circuits and harnesses on the RS. The switched accessory circuit harness is tied to the upper left suspension support, behind the rear trunk wall. It is intended to power the BRP optional cigarette outlet, which is intended to be installed on the left rear trunk wall. For a GPS this may be impractical, but you could extend the GPS cable out of the trunk lid. After installation, which is easy with the existing connector and boot (although the trunk should be removed for best results), the circuit must be activated by putting a 3 amp fuse in the appropriate slot in the fusebox. There is also a 3 amp unswitched accssory harness and circuit. That harness is tied near the steering and trunk latch. Again, best to remove the trunk for easiest access. Either circuit could be extended to an outlet installed elsewhere.
For mor amperage, or other uses or types of outlets, you would be best to install an auxiliary fuse block, fed directly from the battery with a fused wire. There are switched, unswitched, and mixed fuse blocks available. For a single circuit powered all the time, you could also run a fused harness directly from the battery terminals (inside the bodywork, not the jump start terminals under the seat). There are two types of outlets, the common cigarette lighter type, or the Powerlet outlets (BMW type) which are smaller, make a better connection, and are less prone to having the plug come loose while riding.
Wherever you choose to install the outlets, make sure you have enough room behind your panel for the outlet and wiring. A search for "power outlet" should yeild lots of examples.