Today, I finished installing my electrical cutoff switch. I used the kit from Advanced Autosports. You can also get the switch alone from Summit Racing, Pegasus, and others. If you do this, then you will need to get the other parts you need individually to connect to the battery and alternator, label the switch location and off position, etc.

Despite the having instructions that came with the Advanced Autosports kit and a good thread on SpecMiata.com describing the install, I managed to briefly simmer my battery by cross wiring the battery after connecting to the wrong starter terminal. The battery made a disturbing humming sound for a couple of seconds and emitted a sulfur smell before I switched it off. Once I realized my error of having connected to the wrong starter terminal (the top terminal is the one you want), everything worked fine.

Master Switch