I used to have one in a car that I don't have any longer. It more than paid for itself. It saved my behind many a time from tickets. I loved it. If you can take your car to a high end stereo installer who handles these and let them check into it, they'll tell you if it's all there. Otherwise, From what I remember, there's a indicator inside the car somewhere, like on the dash, then there's a knob somewhere else to adjust the volume. Under the hood, there's the actual unit. It sounds like you've found that. The reason I liked mine so much is because I hate taking the car apart everytime I get out of it. I used to take off the stereo face, the radar detector and all my CD's when I'd go to work. With the radar detector built in, I didn't have to worry about it. Plus I didn't have an ugly cord running up to the dash when I was driving. (or worse yet, up to the sunvisor)
Boy, after all this, I guess I need one in my car again...
Kyle De Priest
2007 e320 Bluetec