Wiring up stereo ignition accessory wire

[tegasaurus]

Which wire is the ignition accessory wire? A lot of the body wiring is bad for some reason, and I need to hard wire a 12v accesory to my stereo.
http://www.imagestation.com/picture/sraid157/p4bbfbd9b503249c7ce4eda7162f9826c/f539f81a.jpg

[kandyflip445]

Buy a test light.