Actually, ignore what I said above.
After some more research I believe the trailer wiring is meant to plug into a connector called X393S which is an 18 pin connector located in the loom in the RH rear fuse compartment. The trailer module plugs into the X609 connector in the spare wheel compartment.
Looking at the X393S wiring, pins 3 & 12 are ground, pin 9 is some sort of auxiliary 12v feed, and pins 1, 2, 4, 5, 6, 7 & 8 are for the trailer lights. No idea which is which out of those pins but it wouldn’t be hard to work it out with a test light.