There are three buildable parts in the Tranzit map that you can use to upgrade your bus in Black Ops 2 Zombies. They add some much-needed defenses to your bus which can help you complete several objectives with ease.
The Plow is installed on the front of the bus and instantly kills zombies it comes into contact with. In short, you will be mowing down zombies while driving around the map. The upgrade also makes sure that zombies do not start grouping up in front of the bus before entering from the windows.
The Roof Hatch, if not already obvious, allows you to go to the roof of the bus from the inside. It is installed in the center of the bus, helping you reach vantage points or get a clear view of your surroundings. Just make sure to keep an eye on the zombies as they can enter the bus through the hatch.
The Ladder works similarly to the hatch. It is installed on the outside of the bus, on the left side. This upgrade part shows its usefulness at the Power Plant when you are unable to access the doors on the right side of the bus.
There are four possible item spawn locations in Tranzit, meaning that the bus upgrade parts can randomly spawn in three of them. There is no guarantee that you will always find the same part in the same location.
Bus Upgrade Part #1
The first part can be found right after you build the Turbine on the workbench at the start of the map. Open the door to the right using the Turbine and enter the room. Your first bus upgrade should be next to the right wall.
Bus Upgrade Part #2
The second upgrade part for the bus can be found at the Diner in Black Ops 2 Zombies. Drive over there and then enter the nearby shed to pick up another important upgrade. You will need the Turbine to open the doors here once again.
Do not forget to pick up the Turbine! You will need it again.
Bus Upgrade Part #3
The next location where you can find another bus part is in the Farm area. Exit the bus and head over there on foot to find another shed with a locked door. Hopefully, you have the Turbine because you will need it to open the doors. Head on inside to get yourself the third upgrade for your bus in Black Ops 2 Zombies.
Location for missing Bus Upgrade Part
If any of the three parts do not show up in the three areas mentioned above, you can make your way to the fourth location to find the missing upgrade.
Head back to the bus terminal and enter through the bank vault entrance after blowing it up. Head on in and go straight. Take the first left and then a right to reach a locked door. Use the Turbine here to open the door and enter the room to grab the part you are missing.
