It seems like you've done some initial troubleshooting for the rear defroster issue in your car. Here are a few additional steps you can take:

1. Check the rear defroster grid: When you remove the rear plastic trim to access the connection to the window, inspect the rear defroster grid for any visible damage or breaks. If you find any damaged areas, it may be necessary to repair or replace the grid.

2. Inspect the wiring and connectors: While examining the rear defroster grid, also check the wiring harness and connectors for any signs of damage, corrosion, or loose connections. Ensure that the connectors are securely attached to the grid and that the wiring is intact.

3. Locate the rear defroster control module: The control module for the rear defroster is typically located in the rear of the vehicle. It may be integrated into the body control module or have a separate module specifically for the rear defroster. Consult your vehicle's wiring diagrams or a service manual to identify the location of the control module.

4. Test the rear defroster control module: Using a multimeter, you can check for power and ground at the control module's connectors while the rear defroster is activated. Follow the wiring diagram or specific diagnostic procedures for your car to determine the appropriate voltage and continuity readings.

 

Thanks for the reply, I didn't get time today to remove and check the grid and connections, will do this at the weekend. P.S is there any tips for removing the plastic trim?

 

So todays update.

• Removed the rear plastic trim, with obligatory clips breaking ! (will glue back). Located two single wires left and right, black with spade connector both looked good with no verdigris.
• Put multi-meter on the left wire (looking towards rear of car) other end on metal chassis and pressed demister button and I get 12V! If I perform the same action to the right side though I get no voltage, if I connect my multimeter leads to the left side and the other to the right side I get 12V, If I connect the right lead and go to chassis I get circuit which would indicate that the right side looking towards the rear is ground? can any one provide a schematic?
• There does not look to be any breaks in the window heater.
• The wire on the top of the window is the antenna.
• Connected my iCarsoft and went to menu for testing demister button and it shows its functioning.

I have waited for a while to see if the rear glass feels warmer to the touch, but not sure if this is normal, can anyone advise if there glass feels warm after few mins?

Thanks

 

Problem now resolved, found a loose connection that you cannot access, so fixed it with a $7 product and tooth pick!
video coming soon:

https://www.youtube.com/channel/UCNDYsTCUo6KQ3RU4QLs4OqA

 

Talking about the rear window defroster - here's an other over engineered invention that MB created for us.
First, the usual. You push a button, a request goes to the rear SAM via canbus, the SAM actuates the defroster relay and the red light in the switch comes on. We're used to that sort of electronic stuff.
But there's more to the defroster becoming and staying active. The ambient temp sensor is read for (you guessed it) the outside temperature. The instrument cluster is consulted regarding your speed. The defroster is kept alive for 6 minutes, provided... a few things. When the ambient temp is below +5 deg Celsius, the up time is lengthened by 30 seconds(!) per 1 degree to a maximum of 12 minutes. However, below freezing, your speed becomes a parameter: at 60kmh you get 1 minute extra, a bonus which increases with 1 minute per 10 kmh speed increment to a bonus of max of 5 minutes.
The defroster is cut of if the onboard voltage hits outside the 10,5V-17V range, for more than 70 millisecs. The led in the switch then flashes "at 1,5Hz" (quote). Now, if the onboard tension restores to within the 11,6V-15V range within 30 seconds, the defroster is switched on again. If not, the defroster stays shut of and the blinking led in the switch is turned off.
That's not all. With an ambient temperature below -10 deg Celsius, your request for defrosting the rear window will be accepted gracefully, but the rear SAM will delay the action as long as the interior temp sensor in the overhead control panel reads a temperature of at least +5 deg Celsius. Says WIS: "However, the LED in the rear window heater switch is actuated immediately so that the driver is not irritated".

There's humour in all this. But it also tells us that a failing rear window defroster may be caused by a failing canbus connection, the rear SAM, the dedicated defroster relay, two temperature sensors, the instrument cluster (speed info) and onboard voltage. Or a mundane broken cable. And that a blinking led signals a voltage problem.

 

Thats some great info and makes a lot of sense, I do love over engineered Mercs ! (but not as over engineered as the incredible W124 500E !! . Interesting what you mentioned about the dome light thermistor for the internal temp, mine had broken and I have now replaced last month. So with the control systems all in order, I now have the rear window and everything all in order.

 

Finally finished my video, took me longer to edit than it did to fix the rear window ! LOL. Hope this helps others, welcome to new subscribers.