The CAN and power buffer is very close to completion. Should be non E90 specific and work on any modern BMW's PT-CAN. The connectors and pig tails will make it all completely plug and play without permanently modifying your car's wire harness and is 100% reversable.
On a manual car you'll want to install it at the JBBF or NFRM, on an auto you can also install it at the GWS. Whatever seems best for you. It's going to have the ability to pass through power and will allow you to pull up to 1A from it if it's wired in directly to the module's power supply. If you wire up an aux 12V in, it will allow external devices to pull 5A.
This also isolates the CAN terminations and maintains the car's CAN bus's physical geometries to keep noise and reflections the same as the factory modules are expecting.
Why this is important? Well, here's a scope capture of a CAN bus with a commercially available module wired into it:
Here is the CAN bus after that device was isolated:
I know which trace I'd rather have my car's critical systems running on...