Lester calls either Michael or Franklin to steal a Fire Truck needed for the heist and driving it to the lockup front of the Darnell Bros.. To do this mission, one can simply steal a Fire Truck from a nearby Fire Station or call the emergency services and stealing the truck, either way the police will go after you, so the player need to lose the cops before deliver the truck.
Go to the garnment factory.
Get out of the fire truck.
Gold Medal Objectives
What is your Emergency? - Call 911 to contact the fire department.
Not a Scratch - Deliver the Fire Truck with no damage.
The easiest way to complete this mission is to call 911 on the player's cell phone, then press the button to call the fire department. When the Fire Truck arrives, the player can quickly steal it and drive to the lockup. This method is also required to get the gold medal in this mission.
Try to call the fireman as close as possible to Darnell Bros., this way you can complete the mission a lot more faster and also deliver the truck before the police start to look after you.