With the additional loom you need AMDS7J-19H465-AA, it has 5 connections, A1 A2 A3 A4 & A5. Did you get instructions with this loom?
A2 looks like an earth, A3 and A4 you need to splice up with the Violet/Orange and Grey/Orange respectively. A5 goes to a connector at the front of the car Pin 31 Brown/Orange, A1 then goes to the trailer harness from the towbar itself.
MD Mondeo Trailer Wiring Modification..JPG
MD Mondeo Trailer Wiring Modification 2.JPG
MD Mondeo Trailer Wiring Modification 3.JPG