Look up the wiring diagram for your particular headunit (year, bose/nonbose), and tap the LOC into those four wires (Front Left + and -, and Front Right + and -).
I did it by taking my headunit out, as well as the glovebox, put my LOC behind the glovebox, extended the wires coming from the LOC, and tapped the ends of those wires into the respective wires of the HU's harness.
Try out the Fade feature on our headunit. Set it to R8. You'll hear how just the rear speakers sound. The highs and lows are rolled off (no idea why). So, if you want real bass going to your subs, tap into the front speakers.