Alright boys, here it is! Thanks to ChrisSlicks for ironing out some electrical details.
DIY: Wiring a rear fog light via the BCM and stock wiring harness.
The stock 370Z comes wired for a rear fog light from the dumby light to the BCM in the passenger footwell. What's missing is the control stalk combination switch that relays a momentary signal into the BCM to tell the computer to turn the light on/off. This DIY will help you wire a momentary switch into the BCM to turn the rear fog light on/off. The fog will function as intended by the factory...on with headlights only, off if the headlights aren't on. The catch is that the car doesn't come equipped with the instrument cluster light to tell you if the fog is on or off. To know if the fog is on, I wired a separate light to the 12V power line going to the fog light (coming out of the BCM). So if the fog is on, the 12V light is on too. The light is positioned right by the momentary switch that controls the fog light, which I located in the dumby space between the heated seats.
Necessary parts/tools:
1. OEM or aftermarket rear fog light
2. 22 gauge stranded electrical wire (various colors helpful)
3. wire crimper
4. spade connectors for 22 gauge wire (female and male)
5. (3) two-way 22 gauge wire taps (to tap into the necessary circuits)
6. Dremel and/or drill to prepare dumby slot for switch and light
7. 1N4001 diode to rig in line with the switch wiring (radioshack is your friend)
8. Momentary push button switch of your liking
9. LED or incandescent light to indicate when the fog is on/off (or incorporate in your switch above)
Here goes:
1. Install a rear fog light in place of the dumby as outlined in other DIYs. Plug in the factory harness to the fog light...this will be your power source which is already plugged into the BCM.
2. Locate the BCM on the outside wall of the passenger footwell. The trim pieces snap off, just pull outward on them. There is one screw/rivet that you can pull off deep in the footwell...use a flashlight before you pull off the trim piece and you'll see it. Once this is removed, the side footwell trim can be placed out of the way.
3. The BCM is what's now staring you in the face (multiple wire harnesses plugged into it. It's time to locate the necessary wires for the Input 5 and Output 3 of the combination switch entering the BCM. ***Note, these can also be accessed on the other end of the harness at the combination switch (control stalks by the steering wheel). I found the BCM to be easier to work on and run wires from.
4. Locate Input 5 wire which is BROWN and in the RIGHT LOWER harness entering the BCM if you were facing it head-on. This harness is called M122 and the Input 5 wire is in slot 87. Again, if you're facing the BCM this is the fifth slot up from the bottom in the left column of wires (see photo).
5. Put a wire tap on this wire, thread in the end of a 6-8ft long piece of 22 gauge wire which will run to your switch, and squeeze the tap to join the wires (use pliers).
6. Find the Output 3 Light Blue wire in the LOWER LEFT harness entering the BCM. This harness is called M123. The light blue wire is deceiving as right next to it is a lighter blue wire called SB or sky blue by Nissan. The wire you want is the slightly darker blue which is adjacent to a green wire and the sky blue wire. It's in space 145 of M123 or the 7th slot from the top in the left hand column (again, when facing the BCM). (See picture)
7. Put a wire tap on this wire, insert another 6-8ft 22 gauge wire that you'll also be running to your switch, and squeeze with pliers to complete the tap.
use crimp connectors to connect your 1N4001 diode in line with this wire. The stripe on the diode should face towards the BCM side of the wire.
8. Find the ORANGE wire in the MIDDLE LEFT harness. This harness is called M120 and the orange wire is in space 24. This is the lowest right slot when facing the BCM (and the only obvious orange wire...very easy to see and access).
9. Apply another wire tap and connect another 6-8ft wire that will power the fog indicator light which you'll place by your switch.
10. Finally, take a final piece of wire, crimp on a fork connector, and attach this to the bolt that holds the BCM in place. This is the ground for your indicator light.
11. Run the wires along the underside of the glove box, and then up under the trim pieces along the inside edge of the passenger footwell (under the knee pad and the other side trim piece. You may find it much easier if you remove the shifter trim and take off the knee pad (see separate DIY for these steps).
12. Run the wires toward the cup holder. Lift up on the cup holder to pop it out. It's connected by heated seat harnesses (if you have them), so it won't come all the way out. Fish the wires into this space so you can access the ends, and put spade connectors on all four of them.
13. Pop out the dumby switch piece by gently lifting the tabs on each end. It pushes down out of the cup holder trim for removal.
14. Using a Dremel, grind out all the reinforcing plastic crosswork on the inside of the dumby piece. It's useless and doesn't hurt the structural integrity.
15. Once the inside of the dumby piece is smooth and all the ribs are gone, mark off two spots where you'll drill for your switch and light. The switch and light I chose each required 1/2" holes, so you have to measure twice and get it right the first time. Drill the holes and mount your switch and light to the dumby piece.
16. The switch I used had spade connectors, and I connected these directly to the wire ends leading to Output 3 and Input 5.
17. My light had bare wires, so I attached spade connectors and connected it to the wire tapped into the 12V rear fog power (wire tapped to the orange wire) and the other wire is attached to the ground wire.
18. Pop the dumby piece with light and switch back into the cup holder and reassemble all the trim.
19. Turn on your headlights, and push your momentary switch once. Your indicator light should illuminate and your rear fog light should be ON.
Congratulations. You just wired your rear fog through the factory harness for about $10.
