At the mission selection screen in either single player or cooperative campaign mode, press Up, L1, Down, R1, Left, L1, Right, R1, Up to unlock all missions in that mode.
Pause the game and press Up, Down, Left, Right, L2, R2.
Pause the game and press Left, Right, Up, Down, Up, Down, R2, L2.
When playing in two player mode, go to the mission where you have to protect Furious Thug. At some point you will reach a room with a lot of chairs and a jury stand in the front. Look at the walls until you see some pictures. They appear to be the game developers.
When playing in two player mode, at some point you will reach a mission where you start behind a big pile of sand. First, clear everyone on the other side of the sand pile. Then, go back to the sand pile. Get directly on top of it and you will see a ledge beside the pile. Walk on the ledge and keep going until you reach a new area. You are now inside, or close to the entrance of the door.
Mission 10 ( the last level) is just an outgoing mission. All you have to do is eliminate everyone or restrain a few people. Do not worry about stealth. Just take aim and run and shoot. You will not fight Carter and do not worry about trying to get better parts on your gun for the mission.
In every level you are required to arrest a certain number of suspects. Instead of shouting, if you prefer the assault rifle just shoot the suspects four times then shout for compliance. Sometimes they will give up[ others may die. Test this strategy on different people and find which ones may die when you shoot them. Also on any solo or co-operative mission, choose the shotgun and shoot terrorists in the lower stomach region. They will put their hands up immediately, or as soon as you shout at them. If your aim is slightly off, change to tranquilizer (Triangle) and nail them with four to six shots or one magazine. They will fall to the ground, ready to be cuffed.