Yesterday I was messing around in my trunk and found a black box right by the back passenger 6x9 speaker. It had a harness to it also but some how it wasn't plugged. Anyone know what it is or do I need to plug it in. I have a 5th gen by the way.

 

i believe it is the "brain" for the acoustic feedback system.

if you have an aftermarket stereo, it's quite likely the system was bypassed to avoid dealing with the different impedence of the 5th gen's sound system as opposed to most aftermarket stereo components.

 

^^ Yup... he's got it. But it is more than a brain.... it is an inline amplifier for the rear speakers and helps with the noise dampening. There are microphones in the stock speaker which picks up road noise. This module alos inverts the noise frequency and sends it through the speakers to cancel out noise.