Mark Christie runs the Mystery Ghost Bus Tours in Johannesburg, Pretoria and Cape Town. He also runs them in Grahamstown during the festival.
There is a Facebook page for these, showing when the next public tour will be. Private tours for 30 or more can be organised at any time.
These are truly fun. I have done the Johannesburg one twice as a member of the public and once as part of a private group (the latter is actually better if you have a nice crowd of people doing it). This makes for a great birthday party for adults, and a wonderful treat for clients, guests, staff parties etc.