One of the commonly used sources on the 1150GS for a switched supply is the feed wire (green+black) to the rear brake light switch - which can be located in the area of the frame rail above the rear brake fluid reservoir.
I use this to provide a supply for both the Garmin sat-nav cradle and my Autocom intercom

Contrary to jeremyj's comment, I think you do want the power to the cradle to be off when ignition is off - otherwise the cradle terminals would be live at all times - even when hosing down the bike etc

When I switch off the ignition the sat-nav (Garmin Zumo 350) prompts me that external power has been lost and device will power down in "X" seconds unless I press "cancel" to keep using, so it's still useable with ignition off if you wish - eg,at roadside or in petrol station etc.
Cheers................Grizzly