you dude, ADC means Anologue to Digital Comparator or something. it basically will give you a decimal readout of a signal (usually 0 to 255) rather then 1 or 0. it it can be hard to set up beacuse you would likely need to build a transforming circuit.

do you even know what type of signal must be sent out the amp siganl line? is it 5 or 12 volts DC? does it stay at 5-12 volts dc or does it just sent a pulse?

this could be made without a pic. but if you must these are things to consider.

sound to me like all you need is a zener diode and resistor connected between the ac sound signal and a pics input. not sure how much power a deck can produce and not really sure the value or the components, but get one that wont allow more then 5volts and limited current. if the current is higher then extepcted then i would use an isolation relay like the one posted at the top, this will prevent frying the pic. if all you can find is a standard relay that should work too but will be clicking like crazy.


just write a code that is like this:

if pic recives pulse on input line then
turn on amp
endif


the pics input pin will be reciving random pulses as the music plays so youll ne ed to write a workaround like:

if no pulse is recived within 1 minutes then
shut off amp
endif

how do you plan on powering the pic?


google search zener diode, optical isolation relay, rectification

also, using you meter, meaure the ac signal out the deck with the volume all theway up. also measure the amp signal on your car. im not 100% sure but when you meause the amp signal line go from that wire to ground in dc mode. but when measuring the speakers go from speaker LF + to speaker LF - (left front ) in ac mode.