I once ran the battery in front of the engine and the switch right next to it mounted on a plate, then used a shifter solinoid to knock the switch off. Ran some 16ga wire to button switch on the back. Turned the main on and off by hand normally, but in emergency push the button and kills everything.. The next car I did this on was a door car but also made a rollover switch so that during a hard crash while the driver was occupied with more important issues, the car would shut down the engine and most importantly, the fuel pumps!!!!