Wow, nobody knows?
Did I do something wrong?

 

It's likely more that unless someone has the schematics, or have looked for the exact same thing, you are likely best off just using a multimeter and measure it....

 

I'd run a small power distribution block with fuses under one of the rear panels. This is what I do in my trucks to run air compressors etc. I use Blue Sea Systems:
,
but here are a number of fused distro blocks you can get. At that point it's just running a pair of maybe 6-4 gauge wires back the block, based on amperage you expect to draw (+50%). This is usually the way to run things -vs- trying to pull wire from the front of the car, or trying to find a schematic to splice into some circuit that may affect automotive systems based on current fluctuations.
If it's a small draw, then you can connect to the smaller battery (if you have it like the E63s) that is under the rear seats (at least I think we have a battery there lol).