Lifted some pics which may be useful to people to understand what the BMW wiring connectors are which effectively fuse together head unit output, sub input and mid input:
You can see here:
1. The bulge in the BMW loom, which can be hard to find actually on the passenger side as there are many more cables here.
2. The BMW plastic cover for effectively three fused wires which just pulls off. You can see either + or - here for a front channel.
3. The seperated wires. Note the thinner one is the mid, or was in my case (2007 E92).