Where is the belt retention er on a 99 grand am?

If its the 3.4L it has an automatic tensioner. look right under the alternator and there is a pulley. That pulley and arm is the tensioner. To release the tension, there is a bolt on the inside of the pulley, put a wrench on it and pull like you are trying to tighten the bolt. The arm is spring loaded and will pull up, releasing tension.