That is a crossover box. It is used to split the stereo channels between the center and other speakers in a 3 or 5 speaker configuration.
It looks like you have a modified 5 speaker harness with it as well. I see a second 6 connector plug which would have gone to the rear speaker fader. But it also looks like it's been modified as there is a white connector on it that does not look stock...it was probably changed to work with an aftermarket head unit.
