Looking for switched power source for USB charger

[sailor60640]

I'm adding a USB port to my '99 885i Tiger to keep my IPhone 5 charged. The USB charger device is similar to the one on http://burnsmoto.com/usb-power-socket-weatherproof.php. When this charger is connected directly to the battery it will draw a very small amount of power (milliAmps). Over time it may drain the battery. I want to connect it to a switched power source.
Does anyone have any recommendations on where I should tap into an existing wire or if even tapping into one is the correct solution.
   

[Mustang]

brake light or tail light with a relay ................

any of the light bulbs in cockpit area will be good enough to turn a relay on

this thread will fill in the blanks for you

http://tigertriple.com/forum/index.php/topic,5255