What to use for a signal wire for amplifier

Hey all. I'm not new to installing audio equipment, but with the way these newer cars are wired I figured it best to ask first. '09 Pontiac G3. What wire should I tap to use for an amp signal wire (also location)? The stock radio (with OnStar) stays on with the key off until I open the door, so I'm presuming that the radio positive can't be used? My car doesn't have ABS, can I just jam a fuse tap in that socket in the under hood fuse box (or will the computer give me a code)?

What's the purpose? From your description, it sounds like you are trying to figure on/off signal that might be separate from power? Yes?

I would multi-meter the power to the radio and run the car through the shut off procedure. You can use a needle an and alligator clip as I bet you will not be able to get a probe into the socket.