Go to the Gearhead vault, site of the main mission in subway town. Take out some bosses on the way to the room near the vault. At the bank offices, there are 7 computers that make sounds from Doom when you touch the keyboards. The final keyboard opens... more
Enter Wellsprings and look at the people's shirts. Eventually there will be a person wearing a T-shirt for Doom, which is a game made by the game developers that make Rage!