Electric fan ignition wire

Hi all,

I just installed an aluminum radiator and dual fan setup in my 74 Barracuda but can't figure out where to connect the wire that is labeled ignition. It is so the fans will only get power when the ignition is on and will shut off when the ignition is off. I've looked at a ton of youtube videos and they all just say to hook it up to an ignition source but nobody actually shows where they hooked it up. Please help.
Author: paneldp2