I found that it was easier to kill the fuel injectors than it was to kill the ignition on my truck. There was a fuse for my fuel injectors. I simply removed it, pushed in 2 spade crimp connections in the fuses place that are crimped to 2 18 gauge wires that lead into the cabin of the truck that then gets connected to a switch. I also added a fuse right at the fuse box to keep things safe.
i've build this
for a start-stop setup, although i have yet to complete the final wireing for the ignition cutoff. the start button works fine and can be opperated with my hand on the gear selector when it's in 1st gear... makes for very relaxed restarts at the traffic light... even on those rare occations where the engine doesn't cut in right away.
i'm planning to interrupt the wire going to the coil, inside the cabin useing a relay... pressing the 'stop' button now puts 12v on a wire wich once connected to the relay will power it and cause it to break the connection long enough to stop the engine. releasing it will restore the connection but when the engine is in neutral it won't restart... simpy useing a button with two fixed positions will enable you to interrupt the connection permanently.
both buttons are powered by the same wire taken after the contact swich so you can't start the car without the key.
the wire to the ignition coil has a lot of current going trough it (about 15A om my car i believe) so i'd make sure you'r switch and wires can handle it ... personally i'll use a relay .. the main advantage is that there's no high currents going trough the buttons and you can use thiner wire. also make sure there's a fuse somewhere in the setup... (i took power from the back of the 12V/cigar lighter plug, as it has it's own fuse).
one good tip my dad gave me is to use suitable connector plugs in a relative accessible place rather than to use fixed connections, so you don't have to take appart half the dash or electric system if you need to work on it later.