Ignore me if this has already been covered.
The harness you bought PIGGY, is a universal harness with a toyota end on it. Just cut it and match the wires up, solder, tape, crimps w/e you want to do.... BUT NO TWIST CAPS....
The blue wires are PWR ANT. and or Pre-AMP Wire. or there will be two (ONE OF EACH).
The factory amp if i recall correctly, (plz excuse my lack of memory) will run off an aftermarket headunit. You have to hook up the BLUE (pre-amp) wire to make it work. (if you use the pwr ant. wire it will only come on when you have the radio playing) (when you switch to CD it will shut off). Anyways the pre-amp wire is what tells the factory amp to turn on.
You can run the aftermarket radio into the factory amp (if i recall) it will just get loud REAL quick with your volume level low.
I believe someone posted a link to others showing there installs step by step. I hope I helped if not well

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Sorry I would get into more detail but I am not a very patient person with audio video systems. I installed them for toooooo many years (8yrs to be exact), and just seems quicker for me to do it then to type it, so I get frustrated easily. (I type with 3 fingers... peck......peck.....peck.....peck)
Hopefully I helped and didn't just burn up board space.
If you need real time help feel free to pm me and I will give you my number and maybe I can talk you through it.
Jeremy