In today's modern 4x4 it is often very difficult to find a suitable source of power that is controlled by the ignition switch. The fuse boxes are often full, difficult to get in behind for the purpose of adding extra circuits, and any spare slots in the fuse box are normally void of wiring, although some may have the 12V rail installed.
So getting 12Volts that is switched on and off by the key for activating a relay to drive an accessory circuit without cutting wiring harness wires is somewhat difficult.
In many of the newer vehicles the cigarette/accessory sockets are not always switched off when the key is off, so having things like laptops or GPS units plugged in could put you in a situation where you end up with a flat starting battery, which is not a good way to be when you're are off the beaten track.
So how do you find a suitable circuit that you can piggy back off to power some accessories, easily. In order to do this you will need to have a volt meter, or voltage tester, and know the basics of using it.
Before you start you need to decide if you want the power source inside the vehicle or under the bonnet and check the appropriate fuse box. When you find a spare slot you need to determine if it has 12 volts available in it. You will need to test the circuit first with the key switched off then again with the key switched on. What you are looking for is no voltage with the key off and approximately 12volts on the battery side with the key on. If you don't have any slots marked as "Spare" then you will need to locate the fuse for something that only works when the ignition is on, this may be Radio, AC, Fan etc.
Having located a suitable fuse slot, you would normally be faced with the dilemma of cutting and splicing wires, which you may not want to do in your new car. That's where these great little gadgets come in.
You simply install your original fuse (if there was one) in the bottom slot, and the desired fuse in the top slot, and plug it into the fuse box in place of your original fuse. This puts your original fuse back in circuit, and also provides you a short fly lead that you can attach your required relay to for a key controlled circuit.