Label & then disconnect the neutal safety wire.(Atuomatics only).Disconnect the positive battery cable. Install the toggle-switch under dash, glovebox or anywhere you think the Badguys won't find it, (If the neutral safety switch is a two wire system, either wire can be cut).Drill a small hole or find an existing one in the firewall (use rubber grommets). And pass the two wires thru it from the switch to your Starter relay, (About 48" long). You only need to cut ONE wire. The small red one. Now, simply splice in the two wires from your On/Off switch. Use the solder & be sure to use shrink tubing to help seal out moisture. That's it! Your done! The car nappers will now be cryin! (Besure to route your wires so they don't interfere with any moving parts while driving).--Remember,..You can use MORE than one switch in series. So if someone finds one switch, they STILL have to find the second, or third!
You can also get a little trick with your cutoff switch and use a on/off button in place of the toggle. If you get stuck along the way, a page in the Haynes manual-5-10, Chapter 5, section 21 has a good picture of the starter relay and related wires. Total time involved; 1 hr. Cost for the Wire, Switch & Shink tubing, under 10 dollars at Radio Shack.