Rear Window Defrost defog On The Blink
 

I have a 1981 240D with a dead rear window defrost.
I would like to start troubleshooting by checking the continuity of the heating element.
So, my question is, where is the connection between the wire harness and the window? How do I axcess it?
My fuses are good and the light on the switch works.
By disconnecting the window I hope to, check for power and check the continuity through the heating element.
Where o where is the connection????
Thank You
Jon J

It's been a while now, but I seem to remember the connections (1 +, 1-) are under the side panels just above the rear seat back. The panels are held down by metal snaps, and must be pulled toward the front and up at the same time - gently.

I have the same problem...

There is a defroster relay as I found it searching data for something else.....

Mine's defrost is dead too. Where is the relay?

be careful when removing the rear parcel shelf in the sedans and coupes.. I must have hit the connections on my defroster and now it blows fuses

The relay in my car is essentially directly behind the leftmost "eyeball" vent in the dash. It is screwed to a brace under the dash. It is accessible with the cluster removed, and also accessible (just less so) from underneath with only the driver's kick panel removed. Look up to see it.

Rear Defrost not working
 

I am trying to get this thread going again.
I have a 79 300D, no rear defrost. Switch works in that it lights up, but it does nothing for the back window. What is the troubleshooting process?
Pop the access panel above the back seat, find the connector and then what? How can I check if the relay is working?

Thanks,
Marty

As a rule, relays "click" when they open and close. With the "ignition" on but engine off, in a quiet environment, try pressing the switch and listen for the click of the relay. No click, no relay.

The relay clicks, but when the window is fogged up, or there is snow outside, it turns off automatically.....and does nothing. Fog and snow stay. What next?

If the relay is clicking on then right back off I would assume the relay is bad.

It is common for the leads going into the window to get corrosion and fail. Unplug them and clean them a little and plug them back in. Worked for me.

When I turn on the rear defroster, a band approximately 2.5 inches in width defrosts across the back. The rest is stone dead. My guess-of-the-moment is that the separation of my back glass layers that is starting at both sides in places is responsible - moisture got in between the layers and corroded the element ends themselves or something.

I'm guessing I'll have to save up for a new back windshield at some point... but a new set of tires has to come first...

The entire grid is one element. Just like series-parallel wired lights, if one circuit stops the others down the path will too but the ones before the break will work fine.

If you can find the break you can solder a jumper across it.

Delamination can allow corrosion of the wires.

Come to Michigan, I've got a perfect one cheap.