Clutch switch? Oh - that switch I removed years ago...
I think you'd be better served with either interrupting the ignition power lead or the fuel pump power supply lead. I might also consider interrupting the CPS lead, but I've not done that, so I'm not sure how it would work, with putting a switch in there (DPST or DPDT, I'm thinking.)
The key element is anonymity - you wouldn't be able to start my truck, even if I told you which keys to use. The kill switches are "hidden in plain sight" as accessory switches, and blend right in. That's not even counting the "extra" switches (I've spent a lot of time in bad neighbourhoods...) that I've got hidden, that I only use when I feel the need to.
If you don't have electrical schematics for everything, I'd be willing to help (I have a 1999 FSM, but no scanner - yet) track things down for you.
Remember that even a kill switch won't stop a determined thief (but it's better than an alarm - even most cops out here think those things are a joke, and I think they're a waste of perfectly good solid-state electronics...) but they have stopped the casual joyrider until I came back out of the store...
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