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Based on my experience and the advice from BaltotheWolf, I don't recommend using the Igniter Fuse method for the Honda FIT. About 15% of the time it causes "dieseling" which prevents the motor from turning off and probably causes damage to the engine over an extended time. I'll update this thread when I have found a new solution for the FIT.
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Just finished my kill switch for my FIT. I don't have a lot of miles on it so I can't say for sure it won't cause trouble, but it uses the ignition coil fuse, which I think is same as key off kill. Anyway, here's how I did it:
Disclaimer: Kill switches are dangerous, I am not an automotive expert, I may have royally f***ed something up with my car, proceed at your own risk.
Parts:
Momentary switch: any, can be low current
Toggle switch: any, can be low current
Relay: Should handle 15A, I used CP1SA-12V-X
Fuse Tap: Got it from Autozone, mini, low profile kind
Wire/solder/connectors: Use what you have
Tools:
Wire Stipper
Crimper
Soldering Iron
Zip Ties
Overview:
Reconfigure Fuse Tap
Wire Fuse Tap to Relay and switches
Locate and remove IG COIL fuse
Install modified fuse tap
Reconfigure Fuse Tap:
The fuse tap is meant to create another circuit in parallel with a given fuse, instead I want it to interrupt the circuit the fuse is protecting (and keep using the fuse for protection) when I cut it with the relay.
Open the fuse tap carefully with a razor blade, I had to remove the heat shrink off the wire.
Remove and flatten the small fuse blade that is meant for the original circuit. This is hard to do without wrecking it.
Solder a heavy gauge wire to flattened fuse blade and reassemble the fuse tap with the wire sticking out. I had to remove bits of the plastic housing with a razor to get it to close properly. I used super glue to hold it together again.
Wire Fuse Tap to Relay and Kill Switches:
Now I had the top fuse position on the fuse tap open to accept the ignition coil fuse and two wires that need to be closed for the ignition coil circuit to be closed. I wired them to the Normally Closed taps on my relay so that if anything happened to my kill switch wiring, it would default to closed. Only a catastrophic relay failure should cause this to disrupt normal function of the FIT.
I put a toggle switch and a momentary switch in series so that I can disable to the kill switch if someone else is driving the car or the kill switch button malfunctions and sticks on.
I installed the relay on the fuse bay door and put the toggle switch in one of the extra button slots near the VSA disable button.
Install Modified Fuse Tap:
I removed the IG COIL fuse and put it in the top slot of the modified fuse tap and installed the whole thing in the original fuse location.
The relay needs a ground connection on the coil side to activate, and I found one that holds the fuse panel in place.
That's it! The install is pretty simple, and should be easy to remove as well.
The toggle switch on the far right is the enable/disable.
And here's the momentary switch on the shift knob.
I'll update this thread after I put some miles on it and verify it works.