After you've installed a stalk with the fog light option and you turn it on the BCM then will fuse and power your new fog lights. The fuses and relay are already in the BCM. You don't need an after market harness with a relay.
Here is a grab from the BRZ Electrical Manual. We need to complete the wiring to the actual fog lights. The wiring is already in the harness currently in the car but rather than digging through the harness under the hood I chose to run two new wires to the fog lights. The better option which is developing as I make this DIY thread is to tap into the OEM harness behind the front bumper at the turn signal connector. I'll cover it later.
If you don't wish to remove the nose of the car the following is an option. I choose it because I didn't want to remove the the front end to find where the wires terminated.
In the schematic above everything shaded in blue is internal to the BCM. We need to tap into the connection points pin 3 and pin 5 of connector C3.
The BCM and this connector are behind the panel under the steering wheel. You remove the panel to the left side of the dash and remove one screw there. There is another screw to the bottom right of the panel that needs to be removed Then the panel just pops loose.
Here is a schematic of the BCU and the location of the pins we are tapping into.
This is a picture of C3 plugged into the BCM. With the red/white (pin 3) and white/red (pin 5) from the schematic above. These are the 12v outputs from the BCM when the fogs are turned on.
Cut and splice into each of these wires and feed them to the out to the fog lights. The red/white to the drivers side and the white/red to the passenger side.
These wires will provide the power side of the fog lights. Find a ground somewhere near the lights to complete the wiring to them.
Flick the switch and enjoy.
