Using the power off the headunit speaker out as Aydu describes is really much more desirable than just switching at the ignition. constant on for an amp without a signal source is not a great idea, shortens life of the amp, and is harder in the long run on the speakers.
Aydu's recommended solution is simple, elegant and the best out there for stock headunit users as part of a new speaker/amp upgrade-
Again-Bravo!