no lights/no power at tow bar

just ready to go hooked van on to car and no indicators/lights,  it was working fine last week,  I checked the vans fuses they were all good,  tried cleaning the pins of the socket, (7 pin) no joy, tried checking car fuses all good, so replaced the socket with a new one no joy,  so bought a prewired socket, stil no joy, even replaced the bypass relay there is power there on entry and exit but nothing at the socket have been trying all day and cancelled our booking,  any ideas of what to do next,  auto eletrications are rare around here.