A 12-volt switch is typically used for electrical accessories in your vehicle. They are easy to install and convenient to use. These switches come in many forms and sizes, but the toggle switch is the most widely used in countries around the globe. You can install your own 12-volt toggle switch so you can hook up a favourite accessory yourself and save some money in the process.
Open the bonnet of your vehicle. Loosen the terminal clamp on the negative battery post with a wrench.
Determine a mounting location for your 12-volt toggle switch, ideally on a flat surface in the vehicle. Remove the locknut from the switch and hold the nut against the flat surface. Mark the centre of the locknut with a pencil to indicate the spot where you will drill a hole. Drill a hole into the surface with a power drill fitted with the correct size of drill bit.
Map a route for running the wires from the battery's positive terminal to the switch and from the switch to the device that will use the 12-volt current. Avoid routing the wires along high temperature areas, such as the engine or exhaust manifold. Install rubber grommets on chassis holes that the wires will go through and keep away from sharp edges that can cut the wires and cause a short circuit.
Measure then cut a length of red wire that will run from the switch to the battery and from the switch to the device or accessory that will use the 12-volt current, such as lights, an amplifier or a cooling fan. Strip off 1/2-inch of insulation from the tip of each wire at both ends with wire strippers. Hold one end of the wire that will connect to the battery and crimp it to the end of a red ring wire connector with a pair of pliers.
Hold the other end of the wire and terminate it by crimping it to a spade-type connector (female) with the pliers. Hold one end of the wire that will connect from the load to the switch and crimp it to a spade-type connector (female). Splice the other end of the connector to the positive (red) wire coming from the load with wire nuts.
Plug the female spade connector at the end of the wire coming from the battery onto a terminal at the back of the toggle switch. Plug the female spade connector at the end of the wire coming from the load onto the other terminal behind the toggle switch. Look at the wires and check the circuit -- the load is wired to a terminal behind the toggle switch, and the wire coming from the battery is connected to the other terminal behind the toggle switch. Mount the toggle switch to the hole and secure it with the locknut.
Cut the wire leading to the battery about 6 inches from the eye terminal with diagonal pliers. Strip off 3/4-inch of insulation from the end of each cut wire with the wire strippers. Splice one stripped wire to the wire attached to one end of an in-line fuse holder by twisting together the stripped ends of both wires. Splice the end of the other stripped wire to the wire attached to the other end of the fuse holder.
Insulate each splice by inserting the end of the twisted wires into a wire nut and screwing it on. Place a fuse inside the fuse holder. The fuse must have an ampere rating that is equal to the ampere rating of the load.
Find a bolt or screw attached to any metal part of the vehicle and loosen it with a screwdriver or wrench. Strip off 3/4-inch from the insulation at the tip of the black (negative) wire attached to the load with the wire strippers. Wrap the stripped end of the negative wire clockwise around the screw or bolt and tighten it.
Unscrew and remove the locknut on the positive battery terminal with the wrench. Slip the red ring connector onto the terminal bolt and replace the locknut. Replace the negative terminal connector onto its terminal post on the battery and turn on the toggle switch to test the device or accessory.