I am looking for a 12 volt source I can tap into that only has juice when the key is turned all the way to the ignition or ON setting. My siren amp is wired directly to my battery, but the thing pulls 5 amps when idle, so for now I have to make sure it is turned off before I park for the night.

I leave it on while driving, because my stock horn is disconnected, and the siren's electronic air horn is used instead. It is wired in such a way that when I press the factory horn, the air horn comes on instead. And when I have my siren turned onto Wail, I can press the factory horn to cycle through the different tones, such as Yelp, and Phaser.

 

Using a relay, (because of the amperage draw you mentioned), use the yellow wire coming from the 68-pin, light-gray plug (C4) at the BCM. The BCM is located on the passenger side of the center console, adjacent to the fuse panel. Again, make sure to use a relay and your existing line straight to the battery, because you do not want to pull that kind of current through the wire mentioned above.