Where to run power cable for hidden amp

i don't feel like I'm understanding your question. With every amp, there is a main feed, which will be the large power wire, and there is a secondary "control" wire, which is typically a much smaller gauge wire. No matter when or how you want your amp to turn on, the larger "main" feed wire will always be ran directly to your battery with an in line fuse as close to the battery as possible, and sized appropriately so the fuse blows before the wire over heats and causes issues.

That being said, I believe you already understand that. So that takes us to the control wire. The amp will always have power supplied to it via the main power cable, but will not turn on until it gets the command, i.e. voltage, from the control power. That can come from wherever you want it too.

If you would like the amp to turn on with the ignition, there will be a wire that goes from your fuse box to where the radio is located, and this will only be hot when the you turn the key. That is the wire that turns on your radio, and if you tie your control wire into that feed, it will also turn on your amp.

Some people have radio's wired into their car, in my opinion, incorrectly. These radios have power coming only from an "always hot" source, and the radio will not respond to the ignition of the car. If that's the case, you will not be able to control the amp by tapping into that source of power, because it is always energized. If that is the case with your car, or you just want to manually control the amp, Simply run a fused wire from either your battery, or a source of power that is always hot through a toggle switch and on to your amp.