I would take a feed off an ignition live into a relay and get that to switch a 12v to the amp.
Ignition live would be leccy windows, low level dipped side lights,
You could alternatively wire the stereo up with the ignition and permenant live seperate (as they should be) and then splice a feed from the ignigtion there.
It doesnt matter which wire you use as long as it switches a 12v current.
What stereo make and model do you have? Does it use high or low level outputs for the amp?
Best tip,