The Underground Tunnels within The Forgotten City, on the ground next to a burned corpse in the second corner of the area (before the irradiated section).
Requires The Forgotten City (SE).
Although the Quest Display will only activate if one gets the good ending, the Helmet can be displayed regardless of what ending the player obtains.
The Underground Tunnels
Location within the Underground Tunnels
The Underground Tunnels map
The Forgotten City on map