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Rear Window Defogger


Roy M

My defogger has not been working for a while and thought this would be a good time to work on it but have run into a stone wall. The best that I can tell no wires on the window are broken.
Have been trying to check the switch but the AIM manual my other repair manual does not show the same as what I have or I don't understand it [good possiblity]. The switch has four wires connected to it. Black - which is ground, Pink - power source, have checked this and has 12 volts when key is on,then the two remaining wires go under console and I am assuming [know this is dangerous]go to the grid. The two colors are purple and blue. Checked these two wires with ohm meter and got zero resistance between them. Also, the purple wire has voltage on it when the lever is pressed forward as if sending power to the grid. After this I am lost.
Are these two wires hooked to the grid at the rear. I know that at the top of the window, the grid goes to ground on each side.
Has anyone run into this problem and can give any guidance.

I have been putting myself though heck trying to do some wire work on my car. Here is what I have learned. There is no substitute for the actual G.M. service manual. There are some awesome color charts in the rear that will show you the individual circuits.

Since this is the year you own, you need to get your hands on the matching manual. In my case, I have been installing parts off of other year models, so it would be crazy for me to buy a book from every year I am after.

The members here have been awesome, and several of them have scanned and e-mailed me the schematics I have been after.
I can help


I was just snagging some wire harness parts out of a 78 or 79 or so model and took a good look at the rear defrost for you. Turns out there is a relay that comes in from the fuse panel and gets engergized by that switch on the dash. Then the massive size like 8 or 10 gauge wire carries the power back to the rear glass.

I would bet you relay had a failure causing it to not work right.

Hope this helps. The relay should be somewhere in the driver side kick panel upper side dash.

Good luck.


I have not taken the time to look the part up or anything, but I really have to wonder if this relay looking thing is not actually a dual purpose timer/relay. In other words, the switch on the i/p just tells the time/ relay when to trip the power on and off to the rear defrost.

Let me know, if you can't find one local I will be happy to ship this one to you. I have no idea if it works, but I can promise that a rear window defogger is about the only option I am not trying to squeeze on to the 69.
Yes, that's what it does so the power does not stay on for a long time if you forget to shut it off. The rear window defogger is a high amp circuit when it is on, so if it was left up to the driver to always remember to shut it off it could be a potential hazard.
Thanks for the information on the timer/relay. Will take a look at it tomorrow and let you know.
Thanks again.
Well, today I looked everywhere and could not find a relay. Looked in some of the catalogs and saw where the 78 model called for a relay.
Again, my has power coming to one post on the switch, one ground wire and two wires running under the console, these are approximately 18 gauge wires. One of these wires just before going under carpet to the rear is tied into two wires, approximately 10 gauge wires. At this point there is no power to these two wires but the other wire has power when the switch in energised. So I am assuming (know that is dangerous) that the problem is at the back glass defogger or the wires connecting to the defogger.
I feel fairly confident that the wiring and the absence of the relay is correct because the defogger did work up until about two months ago.
Any other ideas appreciated.
Thanks for the offer of the relay but don't really think it would be any benefit.
Thanks again.
Well, I am fresh out of ideas. Since this parts car had already been stripped pretty bad I could not really tell you where that relay was mounted. However, it had to have been somewhere not far from the drive side kick panel area.

Good luck.
Roy, let's rethink this.

1. In order for the power to the defogger to "automatically" go off after a period of time, there must be a relay somewhere since the '79 is not an ECM controlled car.

2. If it was working and has now stopped working then something is "broken" - yes? It's probably (and not in order of probability) 1. relay burned out 2. wires are broken or shorted. 3. Fuse 4. Actual defogger element has been damaged 5. on-off switch is bad.

3. You really need to get an AIM or at least a wiring diagram for your car if you don't already. The AIM would show you where the relay is located. If you have the wiring diagram you can then locate each connection along the way and probe for power. Maybe someone here can send you a diagram for a '79.
I have the AIM and it probably shows the relay but I have been looking and can not find it. Will keep digging.
Thanks for the reply.
May not be of help...

...but I noticed a few mornings ago I had my fan on for the front window and noticed my rear window needed clarity, so I flipped the switch for the rear defroster and it turned my dash fan off while the rear defroster was engaged.

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