OK, this just in from Kicker:
They said the factory setup in my 2013 has a dedicated mono signal from the radio to the factory amp, which in turn splits the mono signal to each of the two 8” door subs. They suggested adding an additional aftermarket mono amp (they suggest the 12CX300.1 model) and pull signal from the factory amp in the closest kick panel, and drive both door subs from that aftermarket mono amp. (Same as the factory setup, just adding more amplification).
Having received this information, I'm not really sure how to hook up this extra amp, it sounds like they suggest leaving the factory one in place? It's a much cheaper way to go if all you want to boost are the door subs. Seems the Shelby system philosophy is to depend primarily on a trunk sub for bass.
2013 GT Premium Brembo
MGW, Blow Fish, AM Hood Struts
Tinted, Borla Touring AB, ProCal Tune
Full Shelby/Kicker Audio Upgrade