To activate this easter egg you need to find six bunny parts located in different parts of the map. Interact with the parts to collect them. Once you've collected all six parts, you will be teleported to a zombie nightclub with the blue bunny as the DJ of the party.
There will be zombies dancing and they will start attacking you once the bunny drops the beat. Once you've cleared all the attacking zombies, some more dancing zombies will spawn and start attacking you when the beat drops.
These zombies spawn in three waves, and you must clear all three waves before you can finally open one of the three reward doors on the stage. If you open the door with the best loot by luck, you get a calling card called the 'Grand Prize.'
This guide shows you the locations of all the bunny parts. We also have a video guide for better directional understanding.
Bunny Part #1
The first part is found inside Hotel Room 304. You can access the Hotel Room 304 via zip line next to the Tombstone Soda perk machine directly below the Apartment Rooftop at spawn.
Initially, when you zip line across, you will be in Hotel Room 301. Go into the room directly to your left after landing.
You will find the bunny part on the desk to your right as soon as you enter Hotel Room 304.
Bunny Part #2
The second bunny part is found in the Alley. Go down the Factory Stairs and through the wall with the purple arrow on it.
When you pass the purple arrow, turn right, and you will see the bunny part on the floor next to some tires.
Bunny Part #3
The third part can be found inside the bar on East Berlin Street.
The part is on the seat inside the bar next to the Jugger-Nog perk machine.
Bunny Part #4
The fourth bunny part is in the grocery street across the street from the bar.
The bunny part is on the shelf.
Bunny Part #5
The fifth part is in the Sewer. Get to the Sewer Access via Power Room.
Keep going straight, and you will find the part on the floor by an unlockable door.
Bunny Part #6
The last and final part can be found in the Department Store on West Berlin Street. It is the same location where the Der Wunderfizz perk machine is.
Use the rope to rappel up to the Department Store. The bunny part is on the shelf right next to you.
Zombie Nightclub & Reward
As soon as you pick up the sixth piece, you will be teleported to a different place. This place looks like a zombie nightclub with the blue bunny as the DJ of the club.
You will find a lot of zombies dancing.
Pay attention to the music. As soon as the beat drops, the zombies will start attacking you, as mentioned before. Once you've cleared the first wave, another horde of dancing zombies will spawn.
They will also attack you once the bunny drops the beat. This will happen the third time as well. Elite zombies will also spawn, such as the Disciple.
After clearing the third wave, the barrier blocking the stage stairs will be cleared. You will be able to climb the stage and open one of three reward doors.
The 'Grand Prize' calling card is achieved when you open the door with the best reward (Golden Chest), and that is purely based on your luck. Once you open one door, you cannot open the other two.
Once the timer runs out, you will be teleported back to the city.
The following is the 'Grand Prize' calling card you get.