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Old 10-23-2017, 01:03 PM   #1
Drives: 2012 VW GTI DSG
Join Date: Aug 2015
Location: Marinette, WI
Posts: 3
Enabling the inner taillight bulb to function as a rear fog light

I did a little circuitry modification on my car to give me a rear fog light and I thought I would share how I did it.

I came up with this idea as I was refreshing the bulbs in the taillight assemblies after partially tinting the lower clear sections (signal and reverse lights) and noticed that the same 921 bulb is used in the inner and outer assemblies for the running (inner) and running/brake (outer) lights. I wondered how the outer lights functioned as both a running and brake light with a single filament bulb and randomly found a forum post, which I can no longer find, that mentioned the voltage supply to the outer taillight bulbs is limited to reduce the brightness when the bulbs are functioning as running lights, but are then supplied a full 12v when activated as brake lights.

As the inner taillights use the same bulbs, requiring the same voltage regulation to limit their brightness as running lights and they are not used a brake lights, I looked into how I could supply the driver side inner taillight with a 12V source to run at full brightness to serve as a rear fog light. Utilizing the inner running light as a rear fog light would mitigate the loss of a reverse light and other modifications which are required to integrate and enabled a rear fog light in the OEM manner. This post was crucial in helping me figure out which wire supplies the “left tail light bulb”, which is the inner light as the outer is deductively identified as “left brake and tail light bulb”.

The objective:
Integrate a 5 pin relay, controlled by a toggle/rocker switch, into the circuit powering the driver side inner tail light bulb in a manner that enables switching the voltage source to the bulb between the regular taillight voltage source and a 12V source.

Disclaimer: I take no responsibility for any personal injury or automotive damage incurred by anyone following the modifications detailed below. Please perform at your own risk.

Parts/Tools/Supplies needed:
1. 12V, 5 terminal Relay
2. Rocker or Toggle Switch (3 Pin: Power, Load, Ground)
3. Spade connectors (male and female)
4. 3M Scotchlok Run and Tap Connectors, or equivalent
5. T-20 screwdriver
6. Trim removal tools (not required but helpful)
7. Electrical Tape
8. Flashlight (headlamp is ideal)

Based on the CECM Pin-out noted above, a relay circuit needed to be integrated to the Pin 27 wire in the black pin connector. I first followed the end of this DIY to find the black CECM Pin Connector and remove it. I recall the wire for Pin 27 is yellow with a grey or white strip.

I removed the wire from the pin connector to provide more room to work with it. To do this I modified a small safety pin by cutting off the latching end to create a small 2 prong fork that fit perfectly into the connector to release locking features on the wire end allowing it to be removed.

I cut the wire about 3 inches from the end. I crimped a female spade connector to the section of wire with the end that plugged into the pin connector, and a male spade connector to the section of wire that led into the firewall. Using mating connectors would allow me to revert the wiring back to stock configuration if necessary by plugging them into each other.

At this point I should mention that I thought about using one of the front fog light circuits in the CECM Pin Connectors to supply the power for the rear light, but then settled on tapping into the front 12V accessory port. I may look at those circuits and revisit this at a later date because, if it works, it could be used to limit the activation of rear foglight to only when the front foglights are already on.

For accessing the 12V accessory circuit to power the switch to activate the relay and power the foglight, I referenced this video

I used 3M Scotchlok Run and Tap Connectors to tap ~10” 18ga wires to the black (power) and brown (ground) wires just below the plug that connects to the 12V accessory plug. I then used 2 more Run and Tap Connectors halfway from the end of the 2 wires I had added previously. This results in 2 power leads and 2 ground leads that merge to 1 power lead and 1 ground lead which are tapped into the 12V accessory port plug wiring.

I routed a 3ft length of wire from the center console to the black CECM plug were the Pin 27 wire was cut. A male, spade connectors was crimped to the wire and it was connected to short wire connected to the pin connector. The other end of this wire in the center console had a female spade connector crimped to it and it was connected to terminal 87A of the 5 terminal relay.

I routed a second 3ft length of wire from the center console to the black CECM plug were the Pin 27 wire was cut. A female, spade connectors was crimped to the wire and it was connected to wire leading to the firewall. The other end of this wire in the center console had a female spade connector crimped to it and it was connected to terminal 30 of the 5 terminal relay.

The connections to the relay and the rocker switch where done as follows:

12V, 5 terminal relay:
Terminal 30 = Pin 27 wire to firewall (noted above)
Terminal 85 = Wire to “Load” pin on rocker switch
Terminal 86 = Ground, 12V Acc
Terminal 87a = Pin 27 wire to pin Black CECM Pin Connector (noted above)
Terminal 87 = Power, 12V Acc

On-Off rocker switch (3 pin)
Power = Power, 12V Acc
Load = Wire to Terminal 85 on relay
Ground = Ground, 12V Acc

I use a mini Dorman light up rocker switch which was smaller than one of the center console button blanks. This allowed me to cut a hole in one of the blanks to mount the rocker switch.

This is photo of the inner taillight lit up to full brightness as a foglight. The photo quality isn't the greatest but the light basically lights up the same brightness as the brakelights when the switch is activated.

The way that this circuit is wired, the foglight can be turned on anytime the 12V accessory plug is powers (car off w/ ignition in ON position or car on).

When the regular lights are on, activating the rear foglight will trigger a bulb out warning for that bulb as the relay is disconnecting the circuit from the CECM that is monitored. However, the warning does not persist once the fog is turned off and the car is turned off and then on again later.
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