That amp/sub combo will actually sound pretty good. Are you gonna install it yourself?
Get a line level converter. tap spkr leads (follow directions on the box)
+ (pos) from the bat.
- (ground) solid metal in trunk, or ground from bat.
remote wire from oem amp in trunk (white wire)
run spkr wire from Fosgate amp to sub box and wire subs into the box.
Go for a test drive to see how it sounds, and while your out, pick up some more beer.