Ok, so I replaced my third brake light last weekend and here are a few tips:
1. The bolts/studs are 8mm on my 3/09 build 1er. RealOEM might say 5mm, but they're 8mm on both the old PN light and the replacement light. It will help with a deep socket, but I did not find it necessary.
2. I found it much easier to install by shaving down the bottom "ridge" on the new light. Took an extra minute to do.
3. Have a steady hand with the rotary tool and take your time. It was my first time using one and it came out perfectly; I just didn't rush anything.
4. Move the rubber gasket out of the way on my new light to trim the plastic closer to the edge (you'll understand this when you see what you're working with).
Enjoy! And good luck!
2014 BMW M235i
Mineral Grey/Coral Red/Aluminum
Retired: '00 323i, '06 325Ci, '09 135i, '06 Z4M Roadster