do yourself a favour and go out and buy what they use on road bikes. They bolt onto your handlebars and basically there is a pin which when placed in completes the circuit. If by some bad luck you fall off the pin kills the motor. The wiring s basic you cut one wire going to your keyand join in your new kill switch. When using the kill switch it just acts like another key. I tried the way your doing it and it is a nightmare to wire. Not to mention that you are sending a signal to earth and dfi systems use earth to activate. Not something I would recommend!!
I think the system is closed going to open, the one you are using is open going to closed