You did try removing the facia and reinstalling it first, right? Not the fuse. The front of the head unit.
Even give it a few love taps around the edges an see what happens.
Take the low level signal wires from the plug that goes into the Bose amp. These wires run from the headunit to the bose amp in the boot. I did the same with mine to run my Focal Bomba bp20 active sub.
Converting the ap racing set-up to the standard V8 rear brakes is as easy as doing the reverse. If you want a standard set up with new in box rotors, let me know.
Could just be that the second hand converter you've installed was already stuffed. Hard not to stress after all the work you've done, but at this point I'd call a tow truck and send the whole thing to a transmission joint to sort it out.
My car had a power leak. Looked almost everywhere. I disconnected the front and rear blackvue cameras, the aftermarket sub amp, still no good. I then opened the glove box and disconnected the switch and that did the trick. The bulb had burnt out, but was still somehow drawing current.