Discovery 4 led lights

Hi everyone, ive got a bit of a problem & I was wondering if anyone in the club could help. I have a Discovery 4 66 plate with led lights, i have a swift challenger 2011 caravan, on checking the lights my caravan main back lights will not work, the indicators work fine. The disco did not gave the electrics fitted to the towbar as standard we had to buy a unit from PF Jones, on my disco's dash the green light is showing the trailer connector so why are the lights not working?? 

We tried connecting a 2020 plate mitsbushi jeep to it and all the outside lights work, side led lights on the caravan, main lights & high lights on the top of the caravan are even working.

Are we missing something, is it the caravan or disco causing the problem, do we need a vlm unit or need led resistors putting in the disco? PLEASE HELP.