Note: This glitch only works on the:T-72, T-62, BMP-1, and BMP-2. First get any player safely next to a tank (this works best on the mission Patriotic Defence). Switch to another player and go to the tank, make sure your teammate is facing the side of the tank and is right next to it. Then shoot your teammate he should fall glitched in the tank, heal him (if you can't go prone and go into aim mode, look under the tank and heal your teammate). Switch to him and pull out your knife, stand directly under the gunners position, which is on the left-handside of the turret. Stand up and start to knife in that exact position or maybe try and knife all over the place (in that exact position). Then you should hear a grunt as in the noise a soldier makes when he gets killed, the gunner will be dead and the turret will be inactive. For some reason you can't kill the driver.
Go to the main menu and press and hold the left shift button. Then type desertwatch and go to options. There will be the cheat menu.