Cross the Leisure Park
Go to the leisure park, which is a large outdoor area situated in the center of the mall, by exiting the mall through any external door. An underpass is located on the southwest side that leads to the entrance of the maintenance tunnels. Be careful when heading toward the maintenance tunnels; there are a lot of zombies in the underpass and escaped convicts in a jeep with a mounted machine gun are driving around the leisure park. It is a good idea to kill the convicts before crossing the leisure park.
Use the Car
Get in the red car located outside the ramp that leads down into the maintenance tunnels. The zombies will find it much more difficult to attack you while you're driving. Running the zombies over helps you to navigate through the maintenance tunnels in relative safety. Be careful when driving that you don't hit too many walls, as the car will be damaged and eventually explode. Don't try to navigate the maintenance tunnels on foot; there are just too many zombies down there.
Enter the Maintenance Tunnels
Drive down the ramp and turn right at the bottom. You are now in the maintenance tunnels. Drive through the zombies, but don't go so fast that you can't keep full control of the car. Take the first tunnel to the left followed by the next right. This leads you to a dead end with a black door in the wall at the far side of the tunnel. Park as close to the door as possible.
Locate the Maintenance Tunnel Key
Jump out of the car and head through the door. You are now in the maintenance supply room. Follow the wall on your right until you reach a stack of boxes in the corner of the room. Search in between the traffic cones on the floor to find the maintenance tunnel key, which is hovering just above the floor. Pick up the key and keep it in your inventory. You can now move around the mall using the maintenance tunnels and corridors, making it faster and safer to visit different locations.