What does this plug into?

[Logan97]

2013 ford expedition xl, was a police vehicle at one point. I was looking at the wires behind the radio and noticed that this doesn't plug into anything and that it has an extension on the end that comes off. Does anyone know what this goes to or is it just something leftover from the police use? Ignore the radio wiring in the background, it may not be the cleanest looking but I had to rewire a lot of things back there when I bought this car.