I'm thinking the subwoofer does not have any kind of Amp built in, so he needs an amp to power the speakers in that box.
I think that the easiest way to go is to get a standalone Plate amp from somewhere like Parts Express -
you would loop left and right channel speaker feeds from his stereo through the plate amp,
it taps off the low low frequency, and feeds it to the sub speakers in the box.
Its mean to be wall outlet powered, and looks to be on sale for under $100
I have no affiliation with parts express.
http://www.parts-express.com/dayton-audio-sa100-100w-subwoofer-plate-amplifier--300-802
Or - do like revox says above - return the MTX automotive sub
and get something more like a hi fi/home theater sub ...