I know Ive read something about it before, and I think Ive seen the location...but what needs to be done to fire the brothel quest?

I don't recall any existing brothel questline. Was it perhaps a mod?

I do recall that perhaps the club/cabaret questline was supposed to build up into a brothel, but if that's correct, I don't believe it was ever implemented.

There is also the nightclub chain which has all of two events and doesn't go anywhere. It might look and sound like a brothel.