There are a few threads where this mod seems to have evolved starting with a great how to by pulsar http://forum.miata.net/vb/showpost.php?p=1605205&postcount=1.
Here is how I did it with the advice of this thread, pulsars thread, and referring to the Workshop Manual and Wiring Diagram Manual on my '09 GT. Cars in other countries are wired differently so I only know this works in the USA.
1. Disconnect the Neg. battery wire with a 10mm wrench.
2. Push the release tab on the front of the fuse block cover and remove the cover. (Just noticed a little fuse puller mounted inside the cover - how cool is that)
3. Remove the two nuts on the fuse block. They were so tight it scared me I was afraid of breaking the fuse block. Used a 10mm deep well, but a regular socket would have been fine. A wrench might have done it, though it would have been more difficult. Remove the two wire terminals from the studs; White wires front, Black wire back. Not much slack in the wires, used needle nose pliers to pull the terminals high enough to clear the studs.
4. Remove the two connectors. The small connector is released by pressing the lock in the side facing the rear. The large connector is released by pressing the lock on the white hold down and lifting the hold down as you work the plug off.
5. Push the four locking tabs to release the upper fuse block and lift it up. Still lots of wires attached so you're only going to go a few inches. I used some bungee cords to hold it up out of the way.
6. Remove the cover from the relay block that was under the fuse block by pressing both release tabs. I had a little trouble getting both the release tabs pushed. Chopstick came in handy to push the tabs and not scratch anything.
7. Release the three tabs, pull up and turn over the relay block.
Note: The Violet/White wire that must be energized to make the fog lights work runs out of the enclosure in one harness then doubles back into the enclosure in another harness.
8. Take the tape off the harness leading to the relay block and the harness where the Violet/White wire doubles back, to get some extra working room.
9. Cut the Violet/White wire.
10. Pull back the wire on the relay side of the cut to the relay block. Insulate the other cut end. I used heat shrinkable tubing but electrical tape will work.
Here is a close-up of a well insulated wire courtesy of Berkut13
11. Tuck the insulated Violet/White wire back in the harness, you're finished with it.
Note: To this point this write-up pretty much follows the how-to posted by pulsar.
Note: The Blue/Orange wire on the AIR COND and DEFOG relays is fed from the ON position of the ignition switch via the fuse labeled A/C in the interior fuse panel.
12. Cut the wire tie holding the access panel for the wiring harness and pull it out. Move the relay block out of the enclosure box to gain some working room. Identify the Blue/Orange wire on the AIR COND relay. It is a short jumper wire from the DEFOG relay.
Note: I went through a lot of extra trouble just because I didn't want to make a connection anywhere except on an existing terminal, I did it in the relay block. It's not worth the trouble and risk of damaging the block. The best way to do it is by splicing the wires as shown in these photos by Berkut13.
13. Cut the short Blue/Orange in half and cut the any excess length from the Violet/White wire. Strip 1/2" or so of insulation from all three wires. If you are going to use heat shrink tubing, slide the tubing on one of the wires. Twist the wires together tightly.
14. Solder the wires together. Notice how the solder flows smoothly, is shiny, the contour of the wire is still visible, and the solder has not gone under the insulation. . After cooling clean the flux from the solder splice with alcohol.
15. Insulate the solder splice with heat shrink tubing or electrical tape.
16. Replace the tape that was removed. Replace the relay block into the enclosure and snap it in place. Replace the access panel for the wiring harness and secure it with a wire tie. Snap the cover back on the relay block.
17. Snap the Fuse Block back in place. Hook up both connectors. Put the two terminals back on the two studs and secure them with the nuts. Nuts should be tightened 82.3 +/- 13.2 in lb; or tight enough not to come loose but not so tight you break something.
18. Replace the fuse block cover. Hook up battery. Replace the battery cover.
19. Reset the steering position sensor: Start the car, turn the wheel all the way in on direction, now in the other direction, now back to center (more or less), turn off the car, restart the car.
20. Check your work. You should be able to turn the fog lights on or off with the switch as long as the ignition switch is in the ON position. The fog lights should not come on no matter what you do if the ignition switch is off. Check for collateral damage; make sure everything else still works.
21. That's all folks, enjoy!
Have fun with it. JZN
Doing this on a european 2010 model I discovered the following which can be useful for others that want to do the same mod.
on step 3: as these bolts were so tight it scared me too to break something, I looked a bit further, and discovered that there is no need to remove these bolts (nor the connectors by the way). If you remove the sliding piece of plastic that frees the harness (which is also suggested in step "12. Cut the wire tie holding the access panel for the wiring harness and pull it out.") the entire process becomes much less risky and simpler.
Without disconnecting anything, it is possible to move the relay block outside the plastic box. Once the relay block removed from the box you can work on the cables underneath easily, and as everything else is still connected, you can test, measure, etc...
Anyway, the other 'interesting' thing is that the colors of the cables in my car were different (I can send photo's), but the position is this same.
Third and last: by connecting the 'other wire' from the relay to the 'earth'; I get my DRL without any switching in the car (as I didn't have the fog lights from the factory, I didn't have the front fog light switch inside the car, this 'mod' saved me 80 EUR for the ungrade of the switch assembly).
Thanks for your interesting and useful website.
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28 May, 2012