Once you get out of the car you will have to enter the base and show the guy at the desk your papers. After this look at the desk across from him and through the gate there will be a key on that desk, grab it. Now when you get to the cable car do not immediately get on. Go downstairs and you'll see a gate to the left and in it what look like somewhere a homeless guy would sleep. Open the gate and you'll have access to the nightmare.