I just bought my first car and it’s a beautiful little 1990 Yota! Being that it’s as old as it is, it’s an easy car to steal and I’d be left heart broken if that happened so I’d like to install a kill switch to take some preventive measure. I have all the materials necessary for the job and I know where I want to put the switch itself, but I don’t know what wire to splice into. I’ve heard of a few options, but everyone seems to have different opinions and there are no clear instructions. I was wondering if anyone could tell me the best and
easiest way to do it? What wire should I tap into and where can I find it? Thanks!

 

The way I personally would do it is a battery ground interrupter, similar to a battery disconnect switch on a drag car, but in a hidden location. That way, if wires happen to fall apart there is no direct power to short things out and start a fire.
Otherwise, the main ignition wire is probably what you would want to splice. Find the wire that carries constant, direct power from your main relay to the ignition switch. If you open circuit that wire, nobody's going anywhere with your truck unless they tow it.

There are other options like the starter relay wire, but with this (assuming you have a stick shift) someone could still bump start the vehicle as it only keeps the starter from working.

 

Another possibility is to interrupt the power going to COR. With the "hold" power, or ground line from the VAFM, to the COR interrupted, it might start, but it won't run. As soon as the ignition switch, or it's wires, releasd, it will die very quickly. Like a second or so.

Either put your switch in the B+ line to the COR from the EFI Main Relay, or the Fc line coming from the VAFM, which is the better choice, IMHO. That will permit the "start but not run" set up. Thieves don't want to be sitting there trying to start the vehicle over and over. It attracts attention, which they don't like.
Not only that, but it prevents the bump start option as well. No fuel from the pump, no run, no matter how they try to start it.

Again, all this is just my opinion, for whatever it's worth.
Pat☺