Kill switch

Both those wires going to and from your switch will be +12V.

Neither should be grounded. (Or negative)

One leg of the vehicle fuse socket will be the supply side, and the other the load side.

Test the fuse block (with no fuse fitted) to see which side is supply and which is load. Supply side will have constant 12v while the load side will be 0 with ignition on. 

Test your piggyback to see which leg of the piggy back is direct to fuse by removing the fuse from the piggy back and seeing which side is not switched. I'm assuming the side that has the wire coming out it is switched. 

Fit the piggyback with the switched leg on the load side, and the fuse side on the supply side of the fuse block. You shouldn't need an additional fuse if the switch is after the fuse in the piggyback. 

My diagram is a bit messy but hopefully makes sense. I'm ignoring the fuel pump relay in that diagram. I'm assuming the relay is between the fuse and the pump. 

Hope that makes sense. 

 
 

Yeah that makes sense now

Pretty much just got to identify constant 12v side and the load side and wire them according to the switch to create the the circuit

Killing switches are fine up to a point but if the evil doers have time because the car is away from other people then they will be able to defeat it. I prefer the idea of a tracker system, it won't stop the car being taken but in a reasonable window of time I do know where the car is or last was so the authorities have a bit of a head start in getting the car back for me.

danparry 91 wrote:

It's push button start lexus.
Has alarm ect but doesn't seem to matter,

Recently had x2 nice mercedes stolen the other day without the keys they must of bypassed or something

Many modern cars have push button start systems like that, which work with a keyfob that is transmitting a radio signal.  Get close to the car, the car detects the fob's signal, turns off the immobilizer, and then you press the button and go.

Thieves will carry a radio amplifier that picks up the fob signal (even if the fob is locked away in the house/office) and amplifies it so that the car believes the fob is nearby.  

The way to foil this kind of theft is to lock the fob in a metal box -- google "Faraday cage."

This kind of theft is not a problem for the Jimny, since outside of Japan they do not come equipped with pushbutton start.  The Jimny's immobilizer only turns off if the button on the key is physically pressed with a finger. That is why I say that the Jimny can't be started without the key.


Edit: after @fordem's comment below

Motacilla wrote:

The Jimny's immobilizer only turns off if the button on the key is physically pressed with a finger.
 

The buttons on the key are for the "keyless entry" system and have nothing to do with the immobilizer - the immobilizer function is completely passive and is dependent on the proximity of the key to the immobilizer antenna which is located around the ignition switch/steering lock assembly.

You should be able to unlock the car without pressing any buttons, by inserting the key into the lock on the door and then start the car by inserting the key into the steering lock and turning it, and you can further explore this by removing the battery from the key to disable any electronics in the key and repeating the exercise.

When the key is inserted and the ignition switched on, the immobilizer "looks" for a key id, and if the response it gets matches what is stored, it will allow the engine to be started, if there is no response or no match, the immobilizer will inhibit fuel injection & spark, the engine will crank, but not fire.

 

Thieves are becoming smarter unfortunately.
The saying about making something idiot-proof, but the world constantly invents better idiots comes to mind. 
Immobilizers can be bypassed with the right tools and know-how, and if the OP has a problem with vehicle theft, then a hidden switch for the fuel pump will do no harm. 
A tracker is a good idea too, though, as a fuel pump switch won't stop it getting towed ...