Get one of those pointy little testers that show you whether a wire is live or not. Find a wire that is live when you want it to be (e.g., always, only when the key is on, when the horn button is depressed, etc.).
Use a relay to connect to that wire.
Run the amp off the battery with an inline fuse, through the relay.
I learned this lesson the hard way :lol: :lol: :lol: Long story. Anyway, the relay has two wires that tell it when to be on or off, that's controlled by the wire that you found. And then two more wires coming from the battery pos/neg that run the power, without putting a burden on whatever wire it is that you tapped into.
For example, as it turns you, you CANNOT run halogen headlamps off of the running lights circuit. At least, not for longer than a few seconds before the smoke comes out of the wire.