Before you initiate the final mission in Story mode complete ALL the Outpost missions. You must do this because the Outpost missions can no longer be accessed after the game is completed which will prevent a faction's shop items from being unlocked.
This useful glitch is performed by parking your Vehicle outside of a building that you can enter. Once parked go inside the building and then exit it and your Vehicle will now be completely repaired with full ammunition.
An easy way to get past Gekkos is to shoot five tranquilizer rounds into their legs which will make it collapse and thrash around on the floor allowing you to walk past without being spotted while that is happening.
The following codes will only work after the October 23, 2008 online update has been added to your game. They are then enabled during gameplay by pressing 'Select' to activate the PDA and pressing L2, R2(2), L2, R2, L2(2), R2(3), L2. The PDA screen then needs to be closed and re-opened before entering the code with the D-pad for the effect you want.
Invincibility: At the PDA screen press Up, Down, Left, Down, Right(2) (this code affects both players if enabled in Co-op mode).
Unlock Grappling Hook: At the PDA screen press Up, Left, Down, Right, Up (this code affects both players if enabled in Co-op mode).
Unlock Infinite Ammunition: At the PDA screen press Up, Down, Left, Right, Left(2).
Refill Fuel: At the PDA screen press Up(3), Down(3).
Unlock ALL Vehicles: At the PDA screen press Up, Down, Left, Right(2), Left.
Unlock ALL Supplies: At the PDA screen press Left, Right(2), Left, Up(2), Left, Up.
Unlock ALL Costumes: At the PDA screen press Up, Right, Down, Left, Up.
Unlock ALL Airstrikes except Nuke: At the PDA screen press Right, Left, Down, Up, Right, Left, Down, Up (unlocks ALL airstrikes except the Nuke).
Unlock Nuke: At the PDA screen press Up(2), Down(2), Left, Right, Left.
Here's a great code which is useful to know when it looks like you're going to die. Just before you do die and the screen starts to get fuzzy run past a boundary and press Square, L1, Triangle, O, Left, Right, Square, X, O, X, L1, L1, R2, Square as quick as you can and if you've entered the code correctly the screen will fade back to normal and you will have temporary invincibility. This code doesn't work on missions.
If your vehicle needs repairing or you need more ammo park it outside allied HQ or outpost at any time during the game and go inside the building and then exit and your vehicle will instantly be repaired and rearmed.
To get this lethal vehicle find Eva the mechanic and test drive the custom built monster truck with built in jump jets that she has been working on. When you've run a check-point race you'll be able to purchase the truck from her.