An automotive switch can be a rocker switch or a toggle switch. They turn on and off accessories in your car. They usually are in, or under the dashboard. Some switches have a single pair of contacts. Others have many pairs of contacts. Higher-current switches are large. To get around this, the switches often control relays. Relays can handle higher current, and they can be located under the hood or in other inconspicuous places. One side of the switch is connected to a 12-volt battery source. The other side connects to an accessory or a relay.
Strip 3/8 inch from the end of a piece of wire. The wire should be long enough to go from the fuse panel to where the switch will be located. Push the wire into a spade-lug and crimp it with the crimping tool. Pull on the wire to ensure that it is not going to slip out.
Run the wire under the dash to the switch location. Strip the switch end of the wire 3/8 inch from the end and attach a spade lug as above. If the switch does not have spade-lug terminals, slip the wire through the hole in the terminal, and solder it to the switch. Heat the soldering iron until it is hot enough to melt the solder. Touch the iron to the connector and the wire at the same time. When it is hot enough to melt the solder, apply a small amount to the terminal and quickly remove the soldering iron.
Connect another wire to the other terminal on the switch. Route the other end of the wire to the accessory or relay you want to control. Connect the wire at in Step 2.
Attach the source wire to the fuse block. Use the test light to confirm that the voltage is supplying the switch. Touch one end of the test light probe to the switch terminal and the other end to the nearest ground connection.
Tape the wires around other wires under the dashboard to keep them hidden and out of the way.