Ive got a R1200RT LC, and the socket is directly behind the sat nav area.
Since I was fitting a new screen, I took the fairing panels off directly above and behind the Sat nav area and you will find the power socket cable tied to a bracket with the sat nav lead attached.
I disconnected this and routed the power supply along and through to the nearside speaker void, this way, I can easily gain access by removing the 2 screws of the speaker cover. This saves taking the whole screen and upper fairing panels off at a later stage.
Ive also connected a 12 volt cigarette lighter socket to the original BMW power socket, which I can pull out from a gap behind the little storage locker thing on the nearside.
Hope this helps.