The Underground Car Contact is an unseen character in Watch Dogs.
The Underground Car Contact is an associate of Aiden Pearce who securely stores stolen vehicles for use at a later time. The contact also sells a select number of vehicles. According to Jordi Chin, the contact is a friend of a friend.
When a vehicle is stolen or its info is hacked off of some pedestrians, the information and details for it can be sent to the contact. The vehicle will then be subsequently added to a "library" of owned vehicles. When vehicle registration details are being exchanged over the internet by pedestrians, the information can be obtained and then sent to the contact. The contact will then be able to swipe that car and add it to the player's vehicle library. Any vehicle that is added to the library can be delivered by using the Car On Demand app on the Smartphone. The requested vehicle will be sent to a spot for the player to pick up.