6Al2 Box 12v Keyed source

I am trying to wire a 6 AL box into my 67 barracuda. I tried to wire it to an open fuse, like a tachometer, but those 12v sources turn off when cranking. I know everything works because i wired the small red wire directly to the battery to test it. Does anyone know of a keyed 12v source that hold 10-12 volts while cranking and running? 1967 Barracuda, 6AL2 box, MSD billet distributor, 4 speed manual. Thanks