If you have access to a multimeter, you can quickly narrow it down.
Are you trying to tap into the wire to power something else auxiliary? If that's the case then I would run off the ignition barrel wiring, and use a relay from a dedicated power source if it has moderate to high power requirements.
If somebody hacked your deck wiring and you're trying to fix it, then a multimeter is your best bet.
If you hacked into your harness because you were too lazy to buy an adapter, then shame on you!
Hope that helps!