Does that plug in to the back of the head unit?
Depends on the LOC you use.
The ones that come in the Kicker kit are super simple, they have the same connectors the speakers have.
You just unplug the speaker, plug the line into the LOC, then you run the rest.
(As described by those here and the item description)
Most LOC's have normal wires so you can place them anywhere between the headunit and the speaker.
as LOCs convert the audio to a low voltage you'll benefit from jacking in as close to your end-point/amp and speakers as possible.
Using an LOC not included in the kit will just need dialed-in.
(the wattage put out by a headunit varies by system and I'm sure GM at least had Kicker set it close to what is needed.
harebrained has a nice post on it near the end of this thread
http://www.sonicownersforum.com/forum/t3391/