Yup, defo cut the right cable in the Sextons side. It is the only black cable and has TWO cables coming out of it.
One is the main thicker one, which does form the HU connector to the car connector.
The other one goes off to the bit for the steering wheel controls, so I presume it must be the earth.
Gimme 2 mins and I will post up a pic
Does the Converter to Car photo look wrong to you Dan?