The master switch must kill the engine and isolate the battery when turned off and must be operated by the driver when strapped into the seat and from an external point that a marshal can reach easily.
Mount the switch where it can be reached by the driver and where you can attach the pull cable. The centre console is a good place.
Disconnect the 2 big cables from the battery +ve terminal and run a battery cable from the +ve battery terminal, through the bulkhead to the switch. Run another battery cable from the switch, through the bulkhead to the 2 cables you disconnected. Make sure you use grommets if you drill new holes in the bulkhead and insulate the connection to the 2 big cables. This isolates the battery.
To kill the engine you will need to break into the ignition wire (blue wire coming out of the connector just above the ignition switch – see the photo on the left) and run 2 x 1.5mm wires to the terminals on the back of the switch.
Left hand drive cars
On left hand drive cars the wires coming out of the connector above the ignition switch follow the colours going in to it. Consequently you need to cut the red wire coming out of the connector just above the ignition switch on a left hand drive car.