Tap into the ignition wire...or the wire that supplies 12 volt in all key positions...could use a multimeter...but if you tell us the model and year we could be exact.
from stock systems, you can sometimes use the "power antenna lead" as a remote. (low amperage 12v switched positive) you have to be careful, though.. on some cars, this only works when the factory unit is on "fm mode", and then the antenna goes down when in "cd or tape mode" good luck you can use the ignition wire from the fuse panel, if you don't mind the amp being on whenever the car is on (regardless of the stereo being on or off)
Installers have a much easier way. Trunk-LOC by PAC It provides a remote turn on for aftermarket amps from factory Head units. It will turn the amp on and off with the radio.
I've installed about 50 or 60 of these units over the years they work great. http://www.pac-audio.com/instructions/TRUNK-LOC.pdf Its been around for a really long time. On direction #6, if I install this LOC behind the radio (and I do most of the time) connect the red wire to any constant 12V source behind the radio.
The one advantage I have over most people here is the fact that I am a professional installer. And I truely admire anyone who wants and tries to things for themselves. Most of what I do and know does not come from books or word of mouth. It comes actually doing it. When easier methods come along that really work, I use it. So if someone needs information and I can provide it, I will. Trunk-LOC was originally made to speed up bass special installs. And many installers would put them in the trunk. (myself included). The problem is that many people would eventually upgrade their HU. That meant more work. So I started putting the Trunk-LOC behind the radio. So radio replacements meant unplug Trunk-LOC, plug RCAs and remote into new HU and you were on your way. Live and learn
