Hidden Ignition Kill switch install - Best wire to put the switch on?

[Yamariv]

Hey Guys, as I invest more and more money in restoring my truck, I've been thinking that I'd like to hide a kill switch somewhere up under the dash. I would like another layer of protection incase I have to park on the street in a shady area.

I want to put a toggle on/off switch but am not sure which wire to put it inline with, any recommendations on which wire to install it on under the dash? I'm looking for the truck to be completely dead if someone tries to turn the ignition.

Thanks!