Trailer connection

[Jputnam150]

Ready to install a 4 pin to 7 pin trailer adapter on 2011 Subaru Forester. Found this black box with no markings. Note there's no 12 gauge wire coming into the harness as would be needed for trailer brakes. What is this box?

 

[brianmoooore]

You can't just connect trailer lights directly to the car lights these days, as you would have in the past, because the car wires are so thin that they already operate at their limit, and there are lamp monitoring circuits that would get upset with additional lamps.
This box takes a feed from the battery, uses the car light circuits to operate relays, and then feeds the trailer lights with the battery power.
The label mentions stop, tail and turn lights, and the four wires on the out side are enough for this.

 

[Jputnam150]

You can't just connect trailer lights directly to the car lights these days, as you would have in the past, because the car wires are so thin that they already operate at their limit, and there are lamp monitoring circuits that would get upset with additional lamps.
This box takes a feed from the battery, uses the car light circuits to operate relays, and then feeds the trailer lights with the battery power.
The label mentions stop, tail and turn lights, and the four wires on the out side are enough for this.

Thank you so much! That makes a lot of sense.