2010 town&country intermittant lighting problem

Have been working with a friend on his 2010 t&c which has a { Hidden Hitch } hitch assembly with wiring harness. Hitch and wiring were installed by a reputable shop and worked fine. He has a utility trailer that was just painted and had some lighting issues,which since have been repaired,but every time he plugs the trailer in now he has no trailer lights untill he first either applies the brakes or turns on a turn signal--any thoughts? :confused:

Lights on the van work ok with the trailer disconnected? If yes I would suspect a bad ground on the trailer.