Rear Defogger Without Factory Harness

[Duner]

I recently finished my DIY rear defrost project for my 2006 LJ, and I figured I would write a guide on how I was able to accomplish this since all the other guides I've seen had different approaches. Here are a few critical aspects of this project.

Key Characteristics:

1. No factory hard top body harness was sourced/replaced, I created my own circuit and spliced into existing instrument cluster wires as needed.
2. This circuit used the internal timer function built into the instrument cluster.
3. This circuit used the factory rear defrost switch.
4. I purchased and built my own relay/fuse box.
5. This was done on a 2006 LJ, but the approach used should work for any year.

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Bill of Material:

1. True Mods Fuse/Relay Box
2. True Mods Power Terminal Block
3. 8 gauge Battery to Terminal Block Wire
4. 10 gauge Insulated Wire for Defroster
5. Terminal Pin Set
6. Jeep Wrangler Hard Top Harness Pigtail Adapter 1997-2002 JEEP to 2003-2006 TOP

I had 12/14 gauge wire laying around for low amp wiring. I ended up using the round tubular pin from the terminal pin set. You can definitely find a better option or reuse one off a parts harness if you have one laying around. I'm sure there are better wire options than what I went with.

The hardtop pigtail was bought since I don't have the body side of the harness. I simply cut the adapter in half and took what I needed.

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Schematics:
The first step to tackling this project was studying the wiring schematics. Personally, I highly recommend you download the service manual for your jeep from here and save them to dropbox, or whichever cloud storage solution you prefer for quick reference on your phone/computer. For this project, I'll be referencing the 2006 service manual. While I am an engineer, it did take me some time to logic my way through the schematics, so luckily for you I did all the leg work. Here is a general list of different components you will be dealing with in the circuit.

• Main Battery Connection
• Accessory Power
• Relay/Fuse Box
• Defrost Switch
• Integrated Defrost Timer (In Dash)
• Rear Defrost Pigtail

There is a lot of information to understand in the service manual, so here is my "watered" down schematic below. I've attached a PDF as well.

• I noted where the original fuse locations were for reference. I moved them all to my own custom relay/fuse box.
• The relay is a 5 pin, which comes with the True Mods kit:
  
  
  
  • C1 = Pin 30
  • C2 = Pin 85
  • C3 = Pin 87a
  • C4 = Pin 86
  • Pin 87 is not used.

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Installation:

Steps:

True Mods Relay/Fuse Wiring:

1. Wire internal connections between relay and fuses in True Mods fuse/relay box.
2. Wire 8 gauge wire from battery to power terminal block, wire connection between terminal block and "Fuse 2" 40 Amp fuse in True Mods fuse/relay box.
3. Connect wire to "Fuse 7" 10 Amp fuse, run it through firewall, and splice into running/ accessory power. I used the cigarette lighter accessory power for this.
   
   
   

Instrument Cluster C2 Splicing/Pinning:

1. Splice instrument cluster C2 lamps feed Pin 6, then wire it into defogger factory switch Pin 4.
2. Splice instrument cluster C2 switch sense Pin 1, then wire it into defogger factory switch Pin 2.
3. Splice/ or pin into instrument cluster C2 pin 7, then wire it into relay pin 85 location in the True Mods relay box. If you already have this factory wire you can simply splice into it. If you need to pin it, like I did, you have to get creative. The kit I ordered happened to have a circular pin with a diameter small enough to snuggly fit around the cluster pin.

Factory Defrost Switch Wiring:

1. Ground switch Pin 1 - I spliced into cigarette lighter accessory ground.
2. Pins 2 and 4 were completed in previous step.
3. Connect pin 3 to "Fuse 15" 10 Amp fuse in the True Mods fuse/relay box.

Rear Defogger C326 Pigtail Wiring:

1. Connect 10 gauge hot wire into True Mods fuse/relay box, then connect the ground wire to a desired location. Since I installed my relay/fuse box in the engine bay, I found it easier to run the ground wire for the rear defrost to this location as well and ground it on the front grill location.
2. Splice into the 10 gauge hot wire and connect it to the "Fuse 15" 10 Amp fuse in the True Mods fuse/relay block.
3. Run the insulated hot/ground wire through the fire wall following the factory harness back to the hardtop connection point.
4. Connect the hot/ground wires to the body hard top pigtail, being wires that connect to Pins 1 and 6 on the pigtail.
   
   
   

Mount True Mods Fuse/Relay Box:

1. I used existing mounting points from the windshield wiper fluid reservoir and cruise control bracket to attached the box to the fender. I then used the two remaining angled brackets to mount the terminal block to the box.
   
   
   

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Testing/ Troubleshooting:

1. With the key in the pre-ignition/ accessory power position, turn your daytime running lights on, and you should see the blue defrost light on the switch turn on.
2. With the key in the same position, press the switch and you should observe the orange light turn on and remain lit. This timer is set for 10 minutes and will automatically shutoff once the limit is reached.
3. While defrost is on, use a infrared, your hand, or any other temperature reading device and observe the grid lines on your rear window. You should start to feel them get hotter and reach roughly 85 degrees Fahrenheit.
4. If your rear defrost temperature does not get hot, there is an issue with either your wiring or the grid lines themselves. Use a DMM across each of the wire "tabs" on the top of the window defrost grid. You should be getting ~12 volts. If you are not, double check your wiring connections/splices. If you are reading 12 volts, like I did, you most likely have a broken grid connection, or a bad tab connection to the grid. The PO did a bad repair job on the tab and the connection was poor for mine, so I opted to swap the whole window with an extra one laying around.