Hey Guys And Gals, Here is my 100% Achievement Guide & Full Walkthrough for Wailing Heights!
Quite a bit into Act 2, you as Soul Ghoulman will be in the police station interrogation room with the zombie that was selling the Class A drug, Garlic.
All you need to do with this, is exhaust every single dialogue option.
This includes all the branches of options, so for instance if you say no, there will be a few dialogue options relating to that. When you say yes, there will be some options relating to that.
So go through it all until it unlocks!
Or go to 1:00:45 in my video to see where and how it unlocks.
Hope This Helps!