Would it be possible for you to use the power wire for the pre-existing amps in the rear speakers to act as the remote turn on of an external amp?

 

I believe you can, but I'm assuming that because its the power wire meaning constant 12v source, your amp will be ON all the time. Why don't you use the remote wire from the pre existing amps instead?

 

Yes,
Use the Purple/Lt.Blue Wire.
This wire connects to #4 on the rear speaker plug.

 

Well I've always notice the pre-existing amps powering up only when I turn on my radio or put in my faceplate. I even had to hook up the antenna power wire from the head unit to a wire to power the existing amps when I installed my stereo.

I had a slight clue it would be possible..thanks!

 

I apologize for claiming this false. The blue wire is the constant power, while the green is the remote wire..and I'm going to use the blue running off of a relay with the green to power a fan for my amp

 

To clarify you want to use the factory amp wire as a trigger for the new amp.
I don't see why you could'nt...