Ok so here is the finished product. Battery feed direct from battery, same with ground.
The brake signal is picked up from the trailer plug, and is routed through the cabin to the controller in some of the black plastic flexible conduit. The brake actuator power lead comes back to the plug through the same conduit.
View attachment 16295 View attachment 16296
When not towing, I can remove the controller from its bracket, and just leave the trailer adapter plug in the luggage compartment.
When swapping over company cars, I just take the whole fuse box cover off, and replace with the original, an disconnect power and earth from the battery. No need to get the the cars 12 pin.