Being a later model it needs the link from the LSM (Light Switch Module) to the passenger fusebox and the link at the engine fusebox.
Got it connected at the rear so ran the cable to the front on the surface to test before hiding it under the carpet and securing the cable with cable ties.
Plugged in a light board and nothing. Sure I connected the link wires OK.
Turn the car around and connect it to the trailer - nothing.
Absolutely sure everything is connected OK. Multimeter out testing and power on the correct pins at the right time - WHAT IS WRONG.
It was then that I saw the earth wire from the socket that I had not connected - testing the socket I earthed the negative inside the car.
Connected the earth wire and the trailer lights worked as they should.
"Don't think of them as problems, think of them as opportunities."
"OK, I think I've hit an insurmountable opportunity!"