There are several places that you can splice in a switch. It needs just a simple A/B switch that will prevent power from following down the wire. I would look for a schematic to find the wire you want and the location to splice into it. It can be the wire going to the starter, fuel pump, or straight to the ECU. I would recommend selecting a wire location inside just to keep corrosion down. If that's not possible use the right connectors with heat shrink, dielectric grease etc...... Sorry that I can't tell you exactly what wire and where it is. I just got my 4th gen last week and the shop manual hasn't arrived yet.
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