This final step can be done anytime, as long as you have inserted all NAVcard and succeeded in all three Easter Eggs for either Richtofen or Maxis: Tranzit's Tower of Babble, Die Rise's High Maintencance and Buried's Mined Games.
Head to the back of the ground floor of the Courthouse in Buried. You will find a 3x4 light grid. This device can be used to track your Easter Eggs. The light in the left column shows whether or not you've achieved the Tower of Babble (on or off) and for what side (blue Richtofen or yellow Maxis). The second column corresponds with High Maintenance and the right column with Mined Games. If all lights in a row show the same colour, one has to shoot the device to reveal a button. Pressing this button will activate the endgame. Congratulations you've got so far!