Fist off, I suggest you just buy a G4. If you cant do the stuff above, maybe a prebuilt is the solution for you.
Also, you dont need a wiring harness. You want to run a dedicated wire to the battery. I realize the pico board will draw little, but those wires are too small and too close to audio. It wont be a good thing once you get it in.
And if you dont want to build an amp, why the hell did you buy a DIY amp? Just buy a regular amp.
You cant get something for nothing. You want an "all in one" solution, you are going to have to either a) buy one or b) solder and drill your own holes.
As for the wiring diagram, you only need to know where the ignition wire is to trip the PSU. So here is what I found with a quick 10 second google: http://www.commandocaralarms.com/wir.../Mazda/Protege