Hold Select and press Right, Right, Left, Left, Square, Square at the chapter selection screen

Entry Location:
2 Story House to the west in Eder Dam

Now this glitch can only be done online and need to be done with 2 people. Ok first what you and your friend do is you both get jeeps (seperate) and whoever wants to get on the roof needs a sniper or a rifle. Then you go in to the alley behind the 2 story house to the westdrive the jeep right up to the wall straight ahead. Then stop and wait for your friend to be accelerating in to the back of your jeep, at this time you need to reverse. Keep doing this until your car is straight up and down then press square to jump out. Then you will be in the best sniping position in the map. Be patient it might take a few trys.