Thanks for the info. I have actually wired in new trailer 7 core wire and plug (seeing the draw bar is also new) all the way back into the van where the original wiring begins. So I know its not that, I checked dozens of times. So next is earth, and good point bacicat that ill check exactly how its earthed to begin with. there is a white wire which feeds to the negative on the battery, so its there.
Ill check this weekend and report back :)

