Get a decent alarm system with 2 way remote, have the alarm shop install one of these:
Amazon.com: Directed Install essentials Dual Zone Motion Sensor 508d: Car Electronics
I used to install these in softop vehicles quite often back in the 80's/90's, they will sense people walking by the vehicle and set off a warning chirp (as long as the alarm has warn-away feature). You can set it to be as sensitive as you like.
If you just wanna do a kill switch, then pull your fuse block down, cut the wire that controls either the ecu or the fuel, then put a toggle switch inline with it. Make sure the switch can handle the current rating of that fuse. If it doesn't, you'll need to wire in a relay too. Not hard, but you'll need a few more connections to complete the circuit.
Personally, I'd rather have an alarm system installed with 2 way transmitter, that way I know when the alarm is going off and can personally check my vehicle! I'd also set it up to not only have ignition kill, but set it up to kill the fuel pump along with the starter disable. Not that hard to do with the newer alarms out there, but its definitely not for the electrically-scared folks to install themselves! You're probably around 40-50 wires that you'll be working with, and they can get very unruly!
I'd suggest researching the following brands for alarms (not in any order):
Python
Directed
Avital
Clifford
Autopage
I have both the Python and Autopage in my vehicles and they work very well. Had a Clifford in another car, but did not care for the way it worked, although they are still a quality brand, just think the others are a bit better. The Python's range is crazy too! I can receive signal from my truck from several blocks away in a busy city (Portland, OR)! The autopage isn't as strong, but still plenty enough for line-of-sight. Basically, if you can see your truck, you'll be able to receive the signal.
I've installed Autopage alarms since 1985 (when the remotes were IR and not RF, and you had to point it directly at a suction cup on your windshield, or you couldn't turn the alarm on or off!), and that's what's in my truck today, but the battery dies much faster than the Python alarm. I think Directed (which owns Python and about a dozen other brands!) has perfected auto security over the past 10-15 years.