Play the last mission Spartan Thorne in episode 5 Memento Mori. Turn around as soon as you start and you should see some rocks, (you might wanna use a scoped weapon) along those rocks, in the middle you will see a radio, shoot that and listen to some music that will turn on for a bit letting you know you got it. After that just proceed with the mission. Post if you just want me to tell you what happens. I won't say it if people want to find out themselves.