Successfully complete the missions to unlock the tank. At the start of the mission, turn left as soon as you begin and follow the path until you see some road blocks. Turn left and go as fast as you can on the path leading up on the right. Get over the wall by driving off the dirt wall at high speed. Once there you will see a drop-off. Drive the tank so you fall down it. Move forward slightly when you get to the bottom. Turn your turret to the left and fire. Make sure to hit the Police truck with your rocket. Then, return to where you fell and press the button for the lift. The lift will rise to the top. Find the dump truck and shoot it. Then, make your way up to the top of that structure. Wait until you are on the inclines before you shoot the Yakuza cars.

An easier way to get the Police Truck is to use the Tank. At the beginning of the level, shoot the dump truck ahead of you. This should blow the dump truck up. Immediately when you see the explosion, press Start then select "Retry". When you do this, you should see the man running. The display and the time will disappear. Go to the left and jump over the ledge. When you are on the green, go up the ramp, turn left and fall down the hole. Aim the cannon towards the Police Truck down the tunnel and fire. Then, go onto the elevator and press the button. Continue following the red arrows pointing up the construction structure. Do not worry about the Yakuza cars, because they do not effect you all that much. Continue on to complete the level and receive the police truck.