I see that some are having issues with this quest as well. My situation seems a bit different.
Before I came to this quest I already went through all the Thieves Guild quests and became the 'Man.' I then completed the Diplomatic Immunity Quest but never got the A Cornered Rat quest.
I can go find Esbern but he wants nothing to do with me (and I don't blame him). What the heck am I supposed to do now?

Does anyone know of a walk around to this bug? Maybe another quest that will intersect or something? Any help will be appreciated.