It depends on what else your going to add to the system. If your going to add the speakers/amplifier I would look into putting it in the trunk with the amplifier. Your going to need to connect it to the amp anyway, so I'm guessing that would be your best bet with a cramped interior. If your going to keep everything else and only add the Clean Sweep, I would put it under one of the seats. It's a pretty small unit and shouldn't interfear with anything. Maybe you could find a place behind the dash too, wich would be the better alternitive for either situation.
In terms of the volume knob, that is the one thing you won't get to look factory. That is most likely going to require cutting into the dash. If I was doing it, the first thing I would try is putting it next to the radio. You know that black surround that goes around the radio? It looks like you could probably fit it beneath the radio on that plastic there. Assuming the radio is the standard single DIN size, there won't be anything behind it and it should fit right in. It shouldn't look out of place there and is right next to all of the other audio controlls so I think thats your best bet.