I'd done this on my past M5 and it was a snap - there was only 1 green wire (carries 12V). When I opened up the panel in the headliner, the wires with the white plastic end was glued right to it so there was no searching required. It was a bit of a PITA to cut away the black wrapping around the wires so there were exposed.
My question/help needed: which wire carries the power? There are 3 different wires that have a green'ish color to them and I picked the wrong one for connecting the blue snap to cut into it. I have more blue snaps ($1.99 from Radio Shack for a bag) but don't want to keep guessing if this is the wrong connector. Pic attached shows white end connector with several wires to it. Sorry for the pic quality, it was my cell phone camera - my digital camera is missing.