Mission 11 - Old Times' Sake
Drive to the objective, a lonely park bench by an intersection. As you wait, a carful of Haitians will pull inside the Louisian Shipping Company Garage.
Go in after them, then duck into cover. Wait for them to split up- the informant will stay in place while the larger man heads upstairs. Take out the third guy, who will come around the right side of the parade float, then sneak up the stairs to take out the other goon. This room also has some money and electronic parts that will be important later.
Head back downstairs and rush the informant, who spills his guts easily enough.
Follow the objective marker to the voodoo shop. There are half a dozen entrances on the ground floor (bad op-sec, mystery crime boss), so take your pick, then make your way upstairs for some more sit-and-wait-action. When the cutscene clears, you and Cassandra will have struck a tentative alliance, and her gun-runner will now be available to you (the red-orange van with the purple ammo icon).
Achievement Alert - Completing this mission also grants you the unmissable achievement 'For Old Time's Sake.'