I have a 4x8 trailer hooked to my Honda Odyssey. They are connected to
each other using a 4 pin connector. The problem I'm having is when the
van lights are "on" then the trailer lights will not work at all.
However, when the van lights are "off" the trailer lights will work.
If I put the hazards on, with the van lights off, all the trailer
lights work, so I know the bulbs and connections work. The turn
signals and brake lights also work if the van lights are off.

I've read lots of places that say double check the ground so to test I
disconnected the ground and the lights still worked when the van
lights were off. I tried wrapping another ground to the van connector
ground and connected it to the trailer hitch but just couldn't get the
trailer lights to come on when the van lights were on. I even bought a
new connector and light and tested with those with no luck. Any ideas
or suggestions are appreciated.
Mark