After Burke sobers up, go ahead and meet him. Meet him in his hideout and after the cutscene, you’ll get the Vehicle Delivery service unlocked. This will allow you to have cars from your garage to be delivered to you anytime, anywhere. By default, you’ll have access to two types of vehicles (excluding the three pre-order DLC vehicles) You can unlock the other locked cars by taking over new districts. The next story mission will also become available.