there is a quest called old soldiers the final part during the final objective (find lost footlocker) you access an attic left of the footlocker is a military radio use this to talk to johnny and get the achievement(with arthur) here's a youtube video of the quest
Just to throw my 2 cents in. The quest was in the north part of St. George's Holm for me. Seems the random generation of maps can put this anywhere in St. Georges. Also It only triggered for me after 9pm. Had to be night to trigger. I did this as Arthur.
Posted by Addington