After completing the "The Missing Parts" mission, go to the Wasted Garage through the back door (the one you used to get out of there). Move forward and you'll reach the room with two destroyed sinks.
Enter the room to the right and keep at its left side. You should find a black cabinet next to one of the walls with red button under it.
Move to it, aim and press E. You should hear the characteristic sound then. Now it is time to find two more buttons.
Move forward. You'll pass by the truck wreckage. There is a room with several big windows near it. Go to the closest one.
To the left you should see another cabinet and right under it (close to the wall) the second red button.
After getting up with the stairs you will see an engine in front of you. Turn then to the right.
You'll get to the red cabinet with the lamp hanging over it.
The last button can be found behind the cabinet. Press it and head to the exit.
On your way back you'll pass by the stairs with the red Quake symbol near it.
Go up the stairs and then walk through the black portal.
Inside the room you'll find the Shambler Plush. It is worth to take it because it very valuable.
In order to leave the room walk on the square rune tile on the floor.