Gotta thank the OP for such a great write-up! Especially the part about the add-a-circuit and the correct fuse to replace! You're a wonderful human being, and I'll fight anyone who says otherwise!
A regular transformer-based Line Output Converter won't work worth crap for a sub-woofer, for the simple reason that transformers suck big time at passing low frequencies.
The ReQ mentioned is one choice - I used an AudioControl LC2i
; oddly enough, the cheapest price was at Crutchfield for an "open box" unit at $89, including shipping.
Data Point: That very tempting 13mm bolt holding the carpet up near the top about a foot in front of the taillight makes a terrible ground. Pull the trunk side carpet back with that bolt out, and you'll see some handy holes in a piece of sheet metal that you can run a bolt through and get a good ground. There's probably no need to ask how I know that makes a terrible ground.
I ran a single blue wire from the add-a-circuit to supply the power to the LC2i and the turn-on power to the amp. Seemed simpler that way.
2011 GT Sterling Grey Premium w/ Stone Leather Interior
Automatic; HID Headlamps
19" Wheels w/ Strut Tower Brace
Comfort Package, Security Package
Rear View Camera, Sound System Upgrades
Magnaflow Catback - now with Resonators!