Discovery back lights come on as soon as engine started

[Spotwash]

Hi all,bought a 2016 discovery sport two weeks ago,last Thursday had a towbar fitted with what I was told was a dedicated wiring kit.Connected up caravan at the weekend and as soon as engine was started the running lights,back lights and number plate lights all came on the caravan even though light switch was off.Got to camp site and unplugged caravan and realised car rear lights are also permanently on.Thought at first they had mixed up the fridge supply which only comes on when engine is running,but that is as it should be.Towbar fitters not interested, any ideas very much appreciated