After T-Bone calls Aiden to say that he found Defalt's "Loveshack", Aiden goes to the Ambrose Theatre to find Defalt. After some hacking, Aiden finds Defalt up in a balcony. Defalt detects Aiden and starts to escape but Aiden either kills him, or downloads his data and let him escape. Either way, Aiden takes the harddrive and escapes as the mission ends.
It is possible to take down Defalt without having to leave the vicinity of the Ambrose Theatre. After evading/killing the fixers, immediately head for Defalt's fleeing car. If correctly performed, a 'Neutralize' indicator will appear as Aiden reaches his vehicle. This will raise the blockers and since Defalt will not leave the vehicle until all data is retrieved, Aiden can just stand next to Defalt's vehicle. However, killing Defalt before the data reaches 100% will result in mission failure.
The car which "Defalt" uses has a unique plate: D3F-4L7.
Defalt's car, the RoseWood, is the only unique NPC car in the game. It is unlocked in the Car on Demand app after completing the mission.
Watch Dogs Walkthrough - Part 193 - Act IV - The Defalt Condition