WASTELAND MISSION 11 - In The Buzzard's Belly
Location: Deep Friah's stronghold - Hope
Reward: Maximum mag sniper rifle upgrade
Objective: Drive into the Northern Tunnel
After hearing about the treasure in the Northern Tunnel from Hope, hop in the Magnum Opus and leave the stronghold. You'll find the marker on the map a fairly short distance to the North-West, in Pink-Eye's territory (Rot 'n' rusties area). As you get near the area, you'll be the target of a sniper or two (unless you've cleared them out), so keep moving as you drive into the Northern Tunnel through the sludge coming out of it.
Objective: Reach the war rig
Wind the magnum opus between the shipping containers and other obstacles. A little further in you should see a wall of metal (shipping containers, wrecked cars, etc) lit up by a lone bulb overhead. In the middle is a metal panel with a red cross on it, showing that you can pull it off with the harpoon. The shipping container's insides that you reveal are covered in spikes, making it necessary to enter on foot.
Note: Even though this is not a collectible location, I will still list the collectibles I found.
You'll see a lock-box full of scrap against a shipping container ahead a few seconds before a swarm (five) of unarmed buzzard enemies appear. It shouldn't take long for you to dismantle them for parts, leaving them strewn across the ooze covered floor. Walk around the shipping container into an open part of the tunnel and another seven unarmed buzzard enemies will rise up from the muck. Bear in mind that three of these will be the equivalent of ferals, leaping at you and using close range unblockable attacks to deal damage.
After the fight ends, check the right side of the area for another lock-box of scrap before moving on. At the back of the open area is a shipping container you can use the jimmy bar on to open up, revealing a path leading further in. On the left on the far side of the shipping container is some scrap in a box.
You can also find some more scrap against the right wall of the tunnel here opposite the rusted bus. Pick it up before you reach the back of the area and find the hauler (war rig) you are searching for. On the ground in front of it is more scrap-in-a-lock-box with another bit of scrap in the ooze beside it.
Take the metal path to the right of the hauler when you are done looting to reach the back of the trailer, which holds the maximum mag upgrade inside. Pick it up. End mission. Leave tunnel.