Where to find ignition wire?

Hey hows it going guys. I will be taking the stereo system out to install a carputer very soon. Anyways I need to find the ignition wire to wire it into my PSU so that when I flip the key it will power on/off.

What would you recommend is the best approach to this? I was thinking I will have to cut the ignition wire and resolder it to my PSU wire.
 
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