I am installing an amp to the factory system as well and also following the same write-up. My 2011 has only one grey box on the right side of the drivers footwell. I haven't looked in the truck. Looking at the wiring diagrams, there was an early and late build in 2011. Anyone know the transition date from early to late? According to the diagrams, if yours is an early build, you will have to splice into the same colored wires, two from each amp. If its a late build, all 4 wires come off of the same amp.
HIS (Mine) 2011 Ford Mustang GT/ Rapid Spec 400A: Performance White/ MODS: Black 18x9 (front)/ 18 x 10 (rear) AM Saleen Replicas with Nitto NT555s, STEEDA Axle-Back, BBK Off-road X-pipe, Barton Industries Short Shifter, 3.73's, BOSS intake manifold, BBK 85mm TB, STEEDA 101mm BOSS CAI, SCT SF3/ BAMA Tuned, JLT Oil Sepator, Eibach Pro-Kit, Adj Panhard Bar, LCAs
HERS (Wife's) 2013 Ford Mustang/ Sterling Grey/ MODS: Flowmaster Axle-Back, K&N Drop-in filter