Most automotive toggle switches are three-prong units that are really quite simple to install and wire up, once you understand exactly what it is you are doing. A well-placed and properly mounted toggle switch will give years of trouble-free service. There are many styles available, from simple paddle toggles to rockers and push-button switches.
Locate a hole at or close to the desired location of the switch, or drill one yourself, making sure it is big enough for the toggle switch to mount into. Be mindful of what is behind where you are drilling, so you don’t short out existing wiring or go through other important components.
Route the wire to be used through the back of the hole, coming out to the switch side, with the terminal connectors already on it. Connect the wire that is to be the hot, or power supply, wire to the appropriate prong. On many toggle switches, the positive side will be bigger, requiring a bigger connector. Do the same with the negative side. Connecting these before the switch is mounted negates the need to fight with them in hard-to-reach places.
Mount the switch into the hole using the supplied nut, which is normally a round, ring-type sealer that screws onto the threaded, male end of the toggle switch. Do your best to make this very snug, because it may be hard to get to at a later date, and also because allowing the switch to wobble in the hole may cause a dangerous short.
Connect the power wire to the battery or a fuse block, making sure the fuses used are the correct amperage for the application. Using the middle outlet on the toggle switch, connect a wire to the accessory to be used, such as lights, an amp, a horn or a cooling fan. Connect the ground wire to the negative side of the battery, or any available good ground, such as screws to the chassis, the frame or sometimes the ground to another accessory.
Test the circuit. When the switch is off, there should be no activity from the accessory, and if tested with a voltage meter, it should read zero at the accessory. When the switch is flipped on, it completes the connection, allowing current to flow from the positive wire to the accessory, activating it.
When drilling a hole for the toggle switch, consider starting with a smaller bit and making a “pilot hole,” to ease the drilling of a larger hole. Route the wires carefully; stay away from extremely hot places and sharp places as well, which could cut the wire and cause a short. If routing the power wire directly to the battery, include an in-line fuse to ensure safety and prevent fires or the destruction of the switch or accessory. If a wire passes through a hole in the body, for instance, install a grommet or heavily tape the wire with electrical tape. A two-prong switch simply gives or cuts power to something; a three-prong switch is for external accessories.