Complete all of the missions related to the Wasted Garage, then enter the location from the fifth floor Radio Tower entrance, and go to the 4th floor. There are three buttons you need to activate, which have no prompt, but do emit a Quake sound once pressed. The first button is in a room by the staircase to the 5th floor, besides a red workbench. The second one is in a small room on the third floor, under a filing cabinet in the corner of a room (crouch and squeeze along the right wall if you have trouble finding it). The third, and final, button is on the second floor, past where you got the Distributor Cap from the vehicle and underneath a nightstand drawer (crouch down and you should be able to activate it). Once all three are pressed, enter the portal near a mattress with the Quake graffiti, which is located on the second floor at the dead end stairwell. You'll find the Shambler Plush item in the secret room, which sells for $150.