Most 12 volt relays operate accessories in a motor vehicle. Applying a small amount, as low as .2 amps of current to the relay coil closes contacts to feed power to an accessory that requires more current to operate, usually over 20 amps. To wire an automotive relay, make at least four connections: two wires to the coil to operate the relay itself and two wires to the relay contacts to apply power to your accessory. You need sufficient electrical skill to select the tools, wire and connectors for the job.
Connect wires to the coil of your relay. This requires two wires from each side of the coil to positive and negative voltage from your vehicle's electrical system. You also need to run either of the wires through a switch to actually operate the relay.
Connect the "common" (C) terminal of the relay to a battery supply. This can be the same supply you use to power the coil. See the reference at autoshop101.com for relay wiring details.
Connect the power input lead from the equipment you want to operate with the relay to the "normally open" (NO) terminal of the relay. When the relay operates, the internal contact will put the voltage on this pin.
If you want to use the relay to turn off equipment when the relay operates, connect the power input lead from the equipment you want to turn off to the "normally closed" (NC) terminal of the relay. Some relays can turn on one accessory while turning off another when they operate. For this type of relay, still connect a power lead to the C lead and the equipment you want to turn on to the NO, and off to the NC contacts. If you use a 12 volt relay for other than automotive purposes, all the same connection principles apply.
If you don't connect to your fuse panel, protect the coil of the relay and any equipment you operate with the relay, by putting in-line fuses in the power feeds you connect to your electrical system.