remote turn on lead

someone at Crutchfield.com told me that i can tap into the factory remote turn on lead for the new amp that i am installing for my sub. How would i go about tapping into the factory installed remote turn on lead?

Where the harness is that plugs into the back of your head unit. I usually run the amp signal wire off one of the head unit ignition wires. Same difference really. All you need it to do is kick on whenever the head unit is on so thats why I feel thats the best one to tap into.