In the cop campaign, you choose a set of objectives labeled as Patrol, Undercover, or Enforcer, and completing those objectives unlocks a new car. Each variation of objectives appears to have a different version of the new vehicle to unlock. For example: completing assignment one as Patrol unlocks an Aston Martin, but completing it as an Enforcer unlocks the same car with a little more emphasis on survivability and more tuned for take-downs.
I can't find a way to unlock the other "flavors" of the vehicles I passed on. Changing Assignment from the menu only lets you change from Patrol, Undercover, or Enforcer among your current objectives. Am I missing something obvious? Besides paying $10 for the DLC "Timesavers" pack?