Hi Neal,
this might be interesting for you:
http://berliner-unterwelten.de/home.1.1.html
the tours are often booked for weeks in advance, but maybe you´re lucky and there´s a chance to pop in. Those "Berliner Unterwelten" tours are very, very interesting and full of history, from WW2 to the cold war.
Viel Spass, Kurfürst