If you don't have electronics under the hood that need continuous power even with the engine off, such as that for a CPU to maintain the memory, then the simplest switch would be one inserted into the positive side of the battery like they use in drag strip applications. I say Positive, or plus, as opposed to the Neg side simply because the negative terminal can have more than one lead returning back to it that could generate a complete circuit to allow the car to run. With positive, it goes right to the starter solenoid and an isolator switch would defeat any attempt to spin the key. As an example, I personally run a master disconnect that breaks the continuity in the Plus battery lead to the starter solenoid. With the switch off, everything is dead, no starter or lights or........................ Switch on, all is well.
When I still ran that miserable misbegotten lump of a FI-Tech EFI unit it needed power even with the key off to keep the memory happy. So all I did was to add a lead to the battery in side of the switch so even with it "Off", the cpu still got its jones for power fed. MPITA because if the unit sat for any length of time it had to have a charger put on the battery or the draw by the CPU would drain the battery.
The only other upgrade to the PLUS side that I included was to go a gauge or two heavier on the cable to account for the longer distance in the power run and the added resistance that the longer cable would possess.
Nick
