I installed an Infinity Basslink II 250w RMS 10" powered woofer in the trunk of my 2010 GT. I ran the remote lead from a +12v I found on the pass side of the car. It's down under the kick panel near the floor. I just kept testing with my multi-meter till I found a +12V on a harness that was on with the key on, and off with the key off. Works good, no popping in the speakers and obviously does not kill the cars battery.
I used a line converter to get the signal to the sub. BUT, I used a good one. It's made by Kicker, but I cannot remember the name of it. It sounds good regardless. I have always had Pioneer head units in the past and used the RCA's so I was a little reluctant to try the line output converter, but it works well. The sub woke my system right up. Not enough bass to wake the neighbors up, but enough to make the radio sound great!
The rest of the system was easy to install. Hope some of this info helps.
BTW- That 2006 Install is a great write up. I used the rear speakers for my signals. And actually, I used the same LOCB.2 he did. I maye re-do it like his. Anyone know if the 2006 SHAKER amp harness is the same as the 2010?
2014 Mustang GT/CS - Coupe