In the map DM-Asbestos, look for a sewer gutter type path. It is near several staircases and it is the place which has a long line of health vials in it. Go through this gutter and at the end of it, you will see a metal fence blocking the way. You can actually walk through this fence and beyond is the redeemer! Note that the Redeener is not always there. It takes a very long time to spawn.