this should be it. i think its the site i used when doing mine
wiring diagram
there isnt a 'amp on' output from the radio or RCA outs. what you have to do is find the 12V ignition wire (a good 10 mins with a multimeter and turning the ignition) and then splice into the rear 2 speakers and wither use a high level adaptior or if your amp has high level in's use them.
before you cut any wires but esp for the speakers use 9v battery test that you def have the right wires.
next time im off ill prob do a quick quide but be a few weeks