The connections are like so:
The 12v pin runs to your trailer battery. The one next to it is for your running lights, so I'm talking about the 2 pins on the side with the "hump". If you run a wire from the 12v to the running lights pins, your tail lights and marker lights will light up on the trailer without needing to be hooked up to a tow vehicle!
I ran some wiring with a switch between these pins, and I can now turn on all the marker lights anytime as long as the camper battery has juice. This is very handy if you need to wander around the camper in the dark, and lets you see without using any bright lights
Using LED marker lights will be easier on the battery, and make sure you don't leave them on too long! My trailer has the 2 tail lights and 11 marker lights that come on, so I only use if for a couple minutes at a time. Great for packing everything up for the night before bed when you put the fire out. And it also doesn't attract mosquitoes and other flying pests to your door for when you get climb in.
I'm sure some of you already know this but I thought this might be handy for those who don't.