And by what you know of how everything is wired, nothing's missing? Maybe they got to a wire while screwing down? Check that the sub is turned on, on the head unit, sometimes they have that setting. See that the ground is on bare metal. Just my case when I screwed down an amp once to an mdf board the screws went past it and were touching metal so it was giving me noise that way. But, can't really say much without being there looking at it.