So I'm probably going to move over by subwoofer from my truck to the FR-S. My question is: how do I get the head unit to tell the amp to turn on and off? A friend who also has an FR-S had his hooked up but his amp stayed on constantly and drained his battery. I know basically nothing about wiring audio (a shop did the wiring for my truck) but I'd like to know how complicated this would be. Also, where is the best place under the car to run the main power line from the battery to the amp?

Sorry for the noob questions.
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'13 FR-S, SBD Turbo kit, Tein Flex Z coils, other stuff. Drifting and autocross.