That's an area i'm really experienced with. I did installation for more than 10 years as now. Still do some times..
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?
100% of the aftermarket amplifier need a constant 12 volt to turn them on. Usally called PCon, Remote or Turn-on Lead. The amplifier will stay on as long as you send him a 12volt on the remote terminal.
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.
Those deck work on 12Volts and never use internal voltage shifter to raise te voltage. So a MAX of 14Volts amplitude will be on the Speaker terminal when the car is running. Simple Voltage divider built with 2 resistor will do the job. Let's say 10K in serie between PIC and Speaker, 5.1k(or a few less) between PIC input and gnd. We can skip the capacitor in serie.