I am almost done with the theivesguild and the dark brotherhood now it seems i am to far to trigger a cornered rat. i know where esbern is but i havnt seen any thalmor and no one has mentioned about the old guy in the basemant. how can i trigger a cornered rat to begin? i have also read it was a glich for the pc i'm on a ps3.

Have you finished Diplomatic Immunity?

It's part of the Main quest. Follow that and you'll be led right to it.

I have the same problem http://www.gamesas.com/user/772279-dynamic1016/. I fineshed Diplomatic Immunity Quest but then a cornered rat quest log never shows up in my menu. ( I am with the thieves guild ). Help me please