Installing Fog Lights to a Fuse Boxby Don BowmanUpdated July 11, 2023
When installing fog lights to a fuse in the fuse box, the amperage must be considered and the appropriate circuit must be used. Fog lights would overload low amp circuits and blow a fuse. First of all, it is not recommended or advisable from a mechanic’s standpoint to run a high amp draw line into the cabin of a vehicle. All high amp draw lights, fans and so on, are activated under the hood with the use of a relay. The reason for this is that anything that draws a lot of amps tends to heat the wire to some extent. A failure of the insulation causing a short will cause wire-melting heat and sparks and an imminent fire.
Most of the switches today are not designed for high amperage. The use of a relay is much safer and very easy to install. The relay for the lights has four posts, and it can be mounted anywhere under the hood. The main power to the lights comes from a larger 10mm wire that offers much less resistance than the normal 18-gauge wire under the dash.
Using the High Beam Switch
There is a 10mm wire that goes from the battery positive terminal to one terminal on the relay and a wire from the relay to the lights. This is the same as a light switch in the house, except that it is turned on and off remotely. The other two posts on the relay are for switched power from a switch in the car, and the other post is simply a ground. The switch in the car takes hardly any power or amps and turns the relay on or off. It is possible to hook the fog lights up to the main fuse and relay box under the hood, although the degree of difficulty is much greater.
Locate the high beam relay for the headlights. Pull the relay out and test the terminals for power with a voltmeter (the key should be off). One terminal will have power all the time. This is the main for the lights. Put the voltmeter on ohms scale and remember which terminal was hot--do not touch this terminal while on ohms or it will blow the fuse in the voltmeter. Check for a ground. When a ground is found, remember which terminal it is. Check the remaining terminals for continuity to the headlight connector. Once that is found, mark it or remember its location in the block, because that is the wire that the new lights will be hooked up to.
Remove the negative terminal to the battery. Remove the bolts holding the fuse relay box to the fenderwell. Cut or remove any device holding the harness to the fenderwell so that some slack will be available to turn the fuse block upside down. Turn the fuse block upside down and remove the bottom section of the fuse block. Be careful not to pull any pins out of the terminals while probing for the wire with continuity on the backside of the terminal. Once found, check with the ohmmeter from the topside to the bottom to be sure it is the right wire (you are looking for continuity). Use a 10mm wire and a piggyback clamping wire connector on the wire for the lights. Install the wire where it can be tied to the exiting harness and replace the bottom cover to the fuse block. Replace the fuse block. Now the fog lights will come on with the lights on high beam.
Using a Separate Light Switch
You can also wire the fog lights to a separate switch in the cabin. Purchase a standard 4-post fog light relay, a 30-amp inline fuse, a remote switch that lights up to show when it is activated, 2 rolls of wire--one 10-gauge and one 16-gauge roll, and a pack of assorted wire connectors. Mount the relay close to the battery. Mount the switch on the bottom of the dash or any convenient place.
Cut one length of the 10-gauge wire to reach from the battery positive terminal to the relay and two lengths of 16-gauge wire to reach from the relay to the switch. Cut another length of 10-gauge wire to reach the fog lights. Put the piece of 10-gauge which goes to the battery positive and one of the 16-gauge wires going to the switch together in a yellow spade connector. Plug this into the battery + terminal of the relay. Cut a piece of 16-gauge wire to go from the ground on the relay to a ground on the chassis or battery negative. Attach a spade terminal on the wire and push it on the ground terminal of the relay. Attach the other end to a ground.
Attach a spade terminal on the second wire to the switch and plug it into the SW terminal to the relay. Put blue spade terminals on the ends of the 16-gauge wires to the switch and plug them into the back of the switch. Connect the 10-gauge wire from the relay to the fog lights positive wire with a yellow butt connector. Attach a yellow connector to the black wire (ground wire) at the fog lights, then attach it to ground. Put a yellow circular wire connector on the last wire that goes to the battery and attach it to the positive post of the battery. The lights can be turned on or off safely at any time.
If the switch is a lighted switch, some light up with the way it is already connected and some need power for the light. If this is the case, then run a 16-gauge wire to the fuse box under the dash and hook it to any fuse that has power only when the key is on. Use a short piece from the switch to a close ground.
Video: How to wire aftermarket fog lights using a 12V relay Toyota Tundra - 2007-2021 Fuse Block Power Wire
Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).