You should be able to get from the boot to the SOS buttons in the center console.
Those have LEDs that come on when the lights are on.
I stumbled across it when I was looking for switched power for somebody installing a GROM.
If you need a schematic, let me know, otherwise I leave it as an exercise for the student:
http://www.bmw-planet.net/diagrams/r.../e85/index.htm
Then again, if you're in the boot, why not the tail lights?