I have always been a little bit in the dark as to how Columbo came to find Brenner. I know it has something to do with the advertising agency where he went to show AJ Henderson's picture. But more than that I'm a little confused at how that leads to finding Brenner at the spot where he is listening to the delfonte speech.
I believe Columbo showed Henderson's advertising agency the picture of Fake Henderson and Brenner from the amusement park (the one where they are together behind the fat lady). Someone there identified Brenner and Brenner's secretary told Columbo Brenner was giving the Dafonte speech.