You must restore Power to the Mall by flipping the Switch located in a dark room towards the back of the Mall, on the upper level by Al's Barber Shop.
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YOU CAN REFER TO YOUR SCOREBOARD INVENTORY SCREEN TO TRACK YOUR PROGRESS OF THE MAIN QUEST

The Gas Mask is located on a Dead Body in the Womens Restroom by the Orange Display Car on the lower level on the Mall.
RESTROOM[i.imgur.com] GAS MASK[i.imgur.com]

On the Gas Mask Body, there is a Note with a Safe Code written on it.
SAFE CODE[i.imgur.com]

You must go to the safe located in the Military Tent near spawn, and follow the instruction manual with your Safe Code to open it.
MILITARY TENT[i.imgur.com]
SAFE[i.imgur.com]

Inside the safe is a Key. Pick it up, and go into the Parking Garage that's located under Spawn and find the FEDRA Armored Truck. In the back of the truck is a crate you can now open with the Key and pick up the Flamethrower.
FEDRA TRUCK[i.imgur.com]

Use the Flamethrower to burn the Fungi & Vines from the Loading Bay Door located by the Orange Display Car.
LOADING BAY DOOR[i.imgur.com]

Inside the Water, there is a Gas Can on a Shelf you can pick up.
GAS CAN[i.imgur.com]

Pick up the Siphon Hose on some scaffolding by the closed off escalators near the shoe store.
SIPHON HOSE[i.imgur.com]

When both parts are acquired, you can go up to the Orange Display Car's Gas Cap and Hold Interact for 5 Seconds while the Gas Can fills up.
GAS CAP[i.imgur.com]

On the upper level by the Barber Shop, there is a red cart with a Generator on it. Facing the Power Room, hold interact of the cart handle to push it forward. Continue holding interact until it comes to a stop and the gas cap opens.
PUSH CART[i.imgur.com]

Fill up the Generator with the gas you siphoned form the Car until that gas cap closes and you hear it power on.

Above the Generator, 8 Lights will now have turned on, displaying various colors. Make note of how many of each color there is.
LIGHTS[i.imgur.com]

Around the Map, there are 8 Breaker Switches with 3 Possible Colors on them. You must match the number of colors on the lights to all of the breakers around the map. (For example: If the display lights show 2 Red, 2 Blue, 2 Purple and 2 Yellow, then 2 Breakers need to be Red, 2 need to be Blue, 2 Purple, and 2 Yellow. Once all the breakers are correct, they will emit a sort of shock sound and your inventory will now have an electric bolt.

BREAKER: NEXT TO MAIN POWER SWITCH[i.imgur.com]
BREAKER: UPPER LEVEL, BY THE FAN ATTIC SHOP[i.imgur.com]
BREAKER: LOWER LEVEL, BY THE DISPLAY CAR[i.imgur.com]
BREAKER: LOADING BAY, TO THE LEFT OF A SHELF[i.imgur.com]
BREAKER: TO THE LEFT OF THE SHOE STORE ENTRANCE[i.imgur.com]
BREAKER: BY THE MILITARY TENT NEAR SPAWN[i.imgur.com]
BREAKER: IN SPAWN TO THE RIGHT OF THE ELEVATORS[i.imgur.com]
BREAKER: JUST INSIDE THE PARKING GARAGE BY THE ENTRANCE[i.imgur.com]

By the crashed plane in the ceiling, there is a hole that will spark electricity in a numeric fasion. Count the number of Sparks you see in order (Short Pause = New Number, Long Pause = Reset). There will be a total of 4 numbers.
SPARK LOCATION[i.imgur.com]

Use these 4 numbers on the Keypad located in the Supply Store to open the storage room.
KEYPAD DOOR[i.imgur.com]

Inside the room, there is a crate of supplies you can pick up (the amount of players in the game will be the amount of supply crates in the room).
SUPPLIES[i.imgur.com]

Pick it up, and bring it to the FEDRA Truck in the Parking Garage. Load them all into the back.

All players must now go to the front of the FEDRA truck and interact to begin jump starting it. [THERE IS NO RETURN FROM THIS]

You must now survive a Lockdown Sequence for 3 minutes until Lt. Shaw says its time to leave. All players will then enter the truck, and you will successfully escape the map.