European Rear Fog Light Activation

The Mk IV GTI's all come with the optics and bulb fixtures to install rear fog lights.  They are not fog lights like the ones up front... they look more like really bright brake lights. There are two ways these fogs can be hooked up.  I started with them hooked up to work along with the brake lights.  They can also be wired to be activated by the European spec light switch.  Last weekend I wired them up so they worked through the European headlight switch.    I may try a hybrid of the two.  It would be nice to have them activate with the brake lights and then be even brighter when they are switched on as fogs.  I'll have to find out if there is some kind of diode that will only let the voltage flow one way so the brake lights don't go on when I activate the rear fogs.

Update!  It works!  I got the rear fogs to work both with the switch in the dash and also with the brake lights.  Click here to see how. 

Below are some photos of what they look like when they're on:

The lower lights below the tail lights are the euro fogs.

The rear fogs when combined with the tail lights are pretty bright.

ECS Tuning has a good set of installation instructions.  According to the laws in the US, it is only legal to activate the drivers' side rear fog.  I activated both for better visibility.  If I get a ticket, then I'll look into disconnecting the passenger side.  


Okay, I did a little research, bought a few diodes, and got the lights hooked up so the rear fogs work both as brake lights and as rear fogs.  The install is quite easy.  This used to be based on the How2 section of 's web site. That site is no longer around. I'll do my best to write up how to do this install.



These are the correct diodes.  I got them for $1.50 at radio shack. 

There are a few main steps to wiring your rear fogs. What I'm describing will allow you to use your rear fogs as extra brake lights and as rear fog lights activated through the euro headlight switch.

The first part of the installation will activate the rear fogs along with your brake lights. This part is very easy.

  1. Cut 2 -8" lengths of 14 or 16GA wire. Attach one of the small female connectors to each end.
  2. Cut each of these wires in half and strip each open end. You'll need to solder a diode in the middle of each of these jumpers. It will look something like what you see below.  

Close-up of Diode Installed.

I soldered it in the middle of the jumper.  Make sure you mark which end has the silver stripe on the diode.  This side must be pointing towards the rear fog lights when you install it.  Otherwise the flow of electricity will not be in the correct direction. 

Pic of the whole jumper.

I used shrink wrap on the jumper to keep it from shorting out.  The dude at Radio Shack said these diodes are low enough amperage that they don't heat up enough to need to be out in the open air.  He said the shrink wrap wouldn't overheat them.  

Final shrink wrapped diode

  1. You're now ready to go to the car. Remove the two interior panels that cover the backs of the tail lights. There are carriers that hold the bulbs that plug into the back of the tail lights. You'll see the wiring harness plugged into them.
  2. Unplug the wiring harness and then remove each of these bulb holders. You'll see two tabs that you squeeze together to release the bulb holder. It takes a bit of wiggling to completely remove them, but be patient.
  3. Install the new bulbs in the blank spot on the bulb holder.
  4. Turn the bulb holder over. You'll see that there's a male spade connector on the back up near the brake light (at the top) and behind the rear fog light (at the bottom left for the passanger side and bottom right for the drivers side). Install the jumper that you made between these two male connectors. Make sure that the silver band on the diode is pointing towards the rear fog light. This will allow the current from the brake light to go to the rear fog, but it won't allow current going to the rear fog to activate the brake light.
  5. Remove the blank cover that is over the hole for the rear fog bulb on the tail lights. Its just a plastic tab that covers the hole. Try not to drop it down into your tail light. It isn't the end of the world if you do, but it will rattle around in there for all of eternity. You can now re-install the bulb holder in the car.
  6. Plug the wiring harnesses back in. Test the brake lights at this point and make sure that the fogs light up with the brake lights. If they don't, then you probably got the diode in backwards. Remove the jumpers and install them the other way.
  7. Unplug the wiring harnesses again since we're going to work with them again.

Now its time to wire up the euro headlight switch.

  1. Remove your headlight switch. Do this by pushing in on the switch part and turn it to the right. You can then pull the whole switch housing out of the dash. Unplug the wiring harness.
  2. Cut the VW wiring harness (the yellow wire with a connector on each end) in half. Strip the non-connector end and solder one to one end of the long 14ga wire. I shrink wrapped the solder joint. Electrical tape will work.
  3. Look at the back of the Euro switch and locate the terminal labeled "NSL". Its embossed on the inside where the wiring harness plugs in. Now look at the wiring harness and find the empty plug over the "NSL" terminal. Plug the wiring connector into that slot on the wiring harness.
  4. Open the fuse pannel inside the drivers door. You should then be able to route the wire down the side through that pannel to the trim along the running board. leave a little slack.
  5. You can then plug the wiring harness into the back of the headlight switch and re-install that.
  6. Route the wire along the door sill. Slip it under the carpet under the rear seat and under the carpet and panel cover till it gets to the rear drivers-side tail light.
  7. You'll need to solder in the third diode into the line here. You'll want the silver line on the diode facing the tail light. This will make it so the LED in the euro switch doesn't light up every time you hit the brakes. ***(Optional: I installed the diode using male and female crimp connectors in case I got it backwards. That way it was easy for me to flip it around if I needed to).
  8. Solder the other end of the VW Connector to the silver band end of the diode. Shrink wrap over the diode to protect it.
  9. Look at the wiring harness for the rear tail light to see where the empty connector slot is. Install the connector that we just soldered in the empty slot. Plug in the wiring harness to the bulb holder.
  10. To activate the passanger side fog with the headlight switch, you need to run a wire from the drivers' side to the passanger side. I found it easier to do this in reverse. Take half of the second VW wires and solder it to the end of a lenght of 14ga wire. You'll need about 6-8 feet. Wrap the solder joint in shrink wrap or electrical tape.
  11. Plug this connector into the blank slot on the passanger side tail light wiring harness. Plug the wiring harness into the bulb holder.
  12. Route the wire from the passanger side to the drivers side. I just threaded it out from the side compartment and wedged it under the plastic molding that covers the back portion of the trunk. I threaded it under the edge of the side compartment on the other side so it met up near the rear tail light.
  13. I used the quick connect here to splice the wire from the passanger side into the wire coming from the headlight switch. I did this BEHIND the diode that we installed (meaning between the diode and the tail light).
  14. Wrap whatever connection you make in shrink wrap or electrical tape and you should be ready to go.
  15. Test out the wiring. If the brake lights turn on the LED in the dash and the readlight switch doesn't turn on the rear fogs, then you've got the diode on the line between the headlight switch and the tails in backwards.

To activate your new rear fogs, you'll just need to pull on your headlight switch. One click turns on the front fogs. Two clicks turns on the rear fogs too.

I'll add some photos to this tomorrow. Hopefully that wil be of some help to you. Let me know if you've got questions.


E-mail me if you've got questions. 

Return to GTI Mod Page

Return to GTI Page

Return to Main VW Roadtrip Page