So looking at the E Learn schematics, the Mic is simply two wires going straight to the B&M module, going through D020. No CAMBus involvement.
So initially I took the Mic out of the lighting cluster and captured the signal. The first picture shows the signal captured when tapping the Mic.
This was to make sure that it is simply a standard cheap passive Mic, requiring no bais voltage to power a preamp or anything like that.
The signal shows a standard Mic signal peaking at around 5mV.
I'm now going to splice into the pins 8 and 24 of the B&M connector and confirm nothing funny is happening before being inputted into the module.
In regards to over loading, it shouldn't be an issue if it is simply a cheap standard passive Mic.
The input impedance of the HU amplifiers is so high you can normally split them three or four times before any noticeable change. So I'm tempted just to see how it performs. I can always just cut the connections to the B&M unit if it is quite or I measure a noticeable attenuation in the signal.
Speaking of impedance, there has been no effort in the wiring to provide any kind of impedance matching at the Mic end and the B&M end. This is all pointing in the direction that the Mic isn't anything special (think the Mic that's integrated into phone headphones).
I'll post the results captured at the B&M end soon! So far it's all looking quite simple.