After I get in with the truck through the doors, What do I have to do ?
I like all the guards, and then ?

Like them?? You have to kill them all! Jump off the roof of the truck and kill them. Tip: quicksave each time you killed one. Then go to the high platform on the other side. Climb the rock and get the secret, walk to the left, down the slope (killing another guard...). Then you come in the next area with a commie to kill...

Happy hunting!
Greetz,
BiK