I wanted to add lights, a louder horn, etc... to my 2012 K1600GTL. Like most who start down this path with a newer BMW, you quickly learn that a switched auxiliary power panel is the way to go. The most common ones I found were Fuzeblock, PMD60, Nuetrino and Centech. I decided to go with the Centech AP-1 as a low cost option, with the intent to spend the extra dollars on the accessories themselves.
The Centech AP-1 is simple, however finding a suitable mounting location on the K1600GTL is a challenge. The GT has a different seat latching mechanism and this makes the space behind the alpine unit a common location for auxiliary power panels, relays, etc... GTL owners are out of luck. Some have opted to mount the PMD-60 on the outside of the "bucket" that is under the seat, and that is fine for a water proof unit. I was a hesitant to do this with the AP-1, even though the vendor says the board is water resistant. After examining the underside of the seat, I found the recess above the battery provided a location with suitable space.
The rear portion of the custom bracket bolts directly to the frame. The front portion rests on the plastic bracket that previously held the BMW supplied screw driver (no loss there if you carry real tools!). Pictures will do more than my words. Hope this is helpful to other K16 DIY owners!