Nothing, the GDR destroyed it twice in the end; the second time as a parting shot, so to speak. The last pictures of it show a couple of rusting iron bedsteads but nothing else and yes, there is a car park on the spot now. However it still feels extremely emotionally powerful to stand in the car park and remember what it was - I was there last September following a visit to Auschwitz and it was a humbling experience. Fitting that such a monster finally came to an end cooped up in an underground cage; mad as a March hare and still convinced it was everyone else's fault!
The amazing thing though is to try to equate the frightening, smoking ruins of Berlin with the beautiful, flourishing city it is now. I recommend it to anyone who really wants to see for themselves what can be done if the will is there.