This Ambush Camp has two entry points. The main entrance is in the southwest end. The “back door” is to the north of the Camp, where you can find the Horse Creek Train Tunnel Research site as shown below. Just follow the train bridge leading to the tunnel, loot the NERO Injector from the hard case there to “clear” the Research Site.
From there, take the uphill path and crawl through the narrow space where there are hanging freakers. One of those are still alive so when it grabs you, break free and kill it.
Drop down from the ledge and take cover. From there, you can Mark enemies and sneak in, taking out enemies from the inside. After getting rid of the patrols, you can now work your way in eliminating the perimeter guards. There are 14 enemies you need to get rid of.
The Bunker’s hatch is located right under the Radio tower. Examine the Map inside to clear this Ambush Camp.