Collect the Death Cards at the following locations to unlock their benefit during Co-op mode.

Thunder
Level: Semper Fi (Eight of Hearts)
Effect: Headshots cause enemies to explode.
Location: At the very beginning after leaving the shack, you will see another shack to the right. The card is inside.

Hard Headed
Level: Little Resistance (Four of Clubs)
Effect: Enemies take less bullet damage.
Location: At the end of the level before going into the last building, look to the right. The card is in a corner of bushes.

Suicide King
Level: Hard Landing (King of Hearts)
Effect: Fire explosive pistol rounds while downed.
Location: About halfway through the mission you will see a big building with a mini-trench outside. Enter the building and go into the lower level. The card is to the right when you go into the room with the stairs.

Cold Dead Hands
Level: Vendetta (Five of Diamonds)
Effect: Enemies take their weapons with them to the grave.
Location: After the sniper section at the first part of the mission, Reznov leads you through a building. Follow the bar around to the end to find the card.

Sticks & Stones
Level: Their Land, Their Blood (Joker)
Effect: Your weapon arsenal consists of a knife and rocks.
Location: Destroy the tanks and go through the barn. Search the stalls to your right to find the card.

Vampire
Level: Burn 'em Out (Queen of Hearts)
Effect: Can only recharge your health by getting kills.
Location: Just before the second mortar position is a bunker to the left. The card is inside.

Flak Jacket
Level: Relentless (Nine of Diamonds)
Effect: Enemies take less explosive damage.
Location: At the end of the mission, take either the left or right path. Eventually the paths will reconnect. The card is about half way down the path.

Body Armor
Level: Ring of Steel (Jack of Spades)
Effect: Enemies die by headshot only.
Location: Enter the asylum, the immediately go to the left. The card is in the far bottom left corner.

Undead Soldier
Level: Eviction (Ace of Spades)
Effect: Enemies come back from the dead.
Location: During the first half of the mission, just after the kitchen area, you will eventually see a room to your right that has a large hole in it. Drop down to find the card.

Painkiller
Level: Blowtorch & Corkscrew (Ten of Clubs)
Effect: Shoot downed Co-Op teammates to revive them.
Location: Go to the left as you move toward the second bunker. A Japanese soldier hanging upside down is near the card.

Berserker
Level: Breaking Point (Three of Diamonds)
Effect: Get three kills in five seconds to become berserk.
Location: Clear all of the four mortar pits, then go to the small shack in the south of this area. The card is inside.

Paintball
Level: Heart of the Reich (Six of Clubs)
Effect: Paintball guns.
Location: The card is in the closed-off subway entrance across the street from the start of the mission.

Victory
Level: Downfall (Two of Spades)
Effect: Limits your HUD, turns on friendly fire, and bleed out in half the time.
Location: Before you get to the theater, some of your allies will break through a door to your right. The card is past that door.