Once you make it down to the basement of the Police Station you will encounter a fellow who has been left for dead in a jail cell. The good news is that this character has a parking card which will allow you to open the doors to the garage and make your way to the sewer.
The bad news is that there’s going to be plenty of zombies between you and that end result because you’re going to need to track down two power panels that will allow you to open the jail cell and retrieve the garage key.
The Power Panel parts are located in two different spots in the game. The first is in the generator room. This is fairly close to where you get the mission. This first part is located in the basement and it’s in a bright orange box that you can see pretty easily.
The second piece is going to be a little bit trickier because you’re going to have to get access to the third floor storage area in the east wing of the Police Station and you’re going to need to get access to the third floor in the west wing of the building to gain access to the Clocktower.
The items that you’re going to need to get to get the second piece of jail power panel will require that you find the Jack Handle which is in the Records Room which is locked by the Club Key and the Large Gear which is located in the third floor storage area of the east wing.
Once you’ve got both of those you’re going to need to head to the Clocktower to solve a puzzle with the gears of the clock and then you’ll be rewarded with the second jail power panel. Both of these pieces need to be taken out of the orange boxes by examining the items then you can apply them to the jail controls to open the bars and retrieve the pass that allows you to leave the garage.