FYI, There's a big brown ground wire that's in another cluster that you can piggy back off of just like the green/white power one, no need to connect it to the bolt.
The Invisicord I had with my GT-R worked in the M3 without a problem and it already comes split into two leads:
Also, I'd recommend using a plain butter knife to unlatch dome unit instead of using your fingers to depress the clips from the windshield. It'll work great and you won't be frustrated by pulling down and possibly deforming the headliner.