Enable 'Cheat Mode' by pausing the game and pressing Up, Down, Up, Down. If the code is entered correctly the prompt message 'Enter cheat code' will appear at the top left of the screen and you will be able to enter one of the following codes to activate it's corresponding effect.
Unlock Invisibility: Enable 'Cheat mode' and then press Up, Right, Left, Down(2), Up.
Unlock Extra Armour: Enable 'Cheat mode' and then press Right, Left, Right, Down, Up, Right.
To get these upgrades you have to this: 1. When you jump out of the plane push yourself to a small tower to your right. Go in there,kill the German, and go down to the bottom floor. One you blow the door turn to your right and there will be swampland and even further to the right will be the beachhead. Run across the swamplands so that you are at the left side of the beachhead. Hidden beneath some debris is the upgrade. 2. In the final mission when you go to the trenches underground I mean completely underground to where ther's a roof(it's not the sniping area!), it will be tucked away in one of the corners(I can't remember where it is though). Both will be the M1 upgrade which also really helps for the sniper area.
To get the weapon upgrades on two of the levels of Market Garden here's what you do: 1. On the first mission right whne you drop, their will be green smokes for your location. One of them will be in a windmill. Drop into it but be careful as their is a crack in the floor which will put you down the next level. Their will be the wepons upgrade there. 2. On the second mission the weapons upgrade is sitting in a corner tucked away in the bunker right before you infiltrate the large bridge. You go into the bunker to blow up the AA-gun and around one of the corners is the upgrade. Both are very valuable as they give you a scope for the Garand.
To get the weapon upgrades on Operation Husky you have to go to these places: 1. On the first mission once you fall out of the plane, you'll notice a large spotlight to your left. Flare your chute and push up to land there. Kill the German and the weapon upgrade will be sitting on the boxes. 2. On Der Bunker, before you go into the Piazza square turn to the first right and keep going until you come across a door to your left. Inside sitting on a desk will be the weapon upgrade. These upgrades give you the special clip to the Thompson and are very valuable.