Where exactly the blue wire should connect to?

"A: This harness emulates plug C on the radio and breaks the signal up into RCA jacks that you can plug into the back of your new head unit. Be sure to properly connect up the blue wire as well, which sends the signal to power up the amplifier." Where exactly the blue wire should connect to?

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[Nick at Pelican Parts]

What year make and model are you working on? What type of head unit are you installing? Typically your aftermarket head unit instruction booklet should have a section relating to the connections to the unit. That blue wire will connect to what is most commonly labeled “remote turn on,” and will likely direct you to connect to the amplifier. If you let me know which vehicle and head unit you are installing I can be more specific. - Casey at Pelican Parts