kill switch

Use a fat relay and wire through the "Am"meter as you suggested, with a power feed to the radio. Hidden relays work, just take the relay with you when you park.

My personal fave: Ground out the MSD or ignition module or coil and put a sizable fuse in the circuit. The car runs without the fuse, but most thieves won't think to pull a fuse to make it run. Blade fuses aren't some thing they can rob from another circuit in our cars, and most people don't carry 'em.

Interrupts to the electric fuel pump work well, too. Especially if you kill a minute before you park. Installing switchery from other vehicles is also possible, just use relays as needed. I'd tell you what I have in my car, but then everybody would do it ;-)