I learned this a while back but thought others might not be aware. If you have to park a popup, travel trailer or 5th wheel in a bad spot at night, you can light up the running lights with the truck unhooked by bridging the pins on the connector with a fuse.
You just bend the tabs on the fuse to match the angle of the connector. but enough different so they hold. Great if you have to leave it someplace dark with possible traffic, and handy if you need to check the running lights or need a little light on the back side of the trailer.