Get on your bike and ride forward.
You'll reach a door. Open it (to the right).
Go to the bottom of the pit using your bike. Watch out, because sections of the road are missing, and you'll have to drive on the wall. Ignore the skeletons in front of you. They won't bother you much :)
At the bottom, you'll have to open this door.
Also a pach of ghouls await you. Throw them a grenade, but don't blow yourself up :)
Now, you'll have to do two things: one is to put the blocks that are in front of the "tower" in their holes (obvious) and second climb up and rotate each section of the tower so that the image on it is in it's right place. This has to be done from TOP TO BOTTOM!!! Otherwise the light won't add up.
Like this. Notice on the floor the sign, denoting the correct position. Do the same for thos beneath you.
You'll obtain this:
Go open the door.