Is that the Subplug 2 harness I've seen?If you do not want to use an LOC, I know that there are people that sell custom harnesses for the xB. They plug into the harness at the back of the stock head unit and provide you with RCAs to run the amp off of. This negates the need to tap into the factory wiring.
Just like BoomBox said...Pop a hole or cut a slit in the big grommet (outer edge) with all the cables going through (drivers side) into the cabin. Just be sure to seal it up real good.Alrighty, there's an area where there's a BUNCH of wires going through, cut a small slit and feed the power wire through there.