OK, I've gone down the wind tunnel. Then i went back up through the caves and back into the Imperial Complex. Now I've opened som doors and killed some more guys and now I'm back in the Tie Fighter Hangar. There is a window on the top floor and also a window in one of the corridors where I can see an officer with his back turned. But I don't know what to do next, or where to go. I think I've been everywhere. PLEASE help, I'm really getting into the game but I'm blowed if I know what to do next.
Sounds like you did everything right, once you come through the tunnels you should be able to find a way into the room with "the guy with his back turned". Just make sure to explore all the rooms carefully after coming through the tunnels.