Where to run power cable for hidden amp

I am wanting to run an amp under the seat of my Swinger to power a pair of 6x9 in the back ( I recently purchased a new package tray with slits for sound). I don't want to interfere with my stock radio in any way.
To power it I know I need to run it up to my battery with a fuseable link. My question is will the amp be off when I don't have a key in the ignition or do I need to interface an on/off switch in the power line? I saw a video on YouTube where a guy ran his power cable to the battery but his amp shut off when he took the key out. Any help is appreciated.