This mission unlocks after you take over the outpost Valsa Docks.
Map - Tagging The Past
2) The first marker
3) Underground descent
4) The second corpse
5) The third corpse
Go to the hut on the peak of the hill . After you receive an assignment from Tanga, You should first go to the marker to the south . Find the descent shown in the screenshot  take the descent.
Once down there, turn and climb the stairs. Halfway up the stairs, you will find the corpse, you are searching for, and a dog tag.
The next corpse can be found at the wall around the cannon, on the left.
The last corpse can be found on the left of the cannon . With all the dog tags, return to the questgiver to complete the quest.