H tward,
Welcome to the Heartland Owners Forum. There's lots of useful information here along with a great bunch of friendly and helpful people.
Here's a diagram from the 2014 Owners Manual that comes with the trailer. Yours is likely the same, although I wouldn't put much faith in the colors. I've also attached a pdf version.
Btw, when you say "When it's dark and the marker lights turn on...", I assume you mean you're towing and the tow vehicle lights come on, turning on the marker lights. There aren't any sensors on the trailer that turn the marker lights on and off.
I would start checking at the junction box under the pin.