Rear window defrost not working on drivers side and possibly drivers mirror.

[Snakedoctor]

It’s starting to get a bit colder in FL and I got in my truck a few mornings ago and headed out. My mirrors were wet and I couldn’t see much so I turned on the rear defrost to activate the heated mirrors as well.

The passenger side mirror heated up quick and cleared and the drivers side just remained unfazed it seemed. The rear glass also started clearing but only on the right side and the slider, nothing on the left side. I’ve read the drivers mirror can take longer to heat because it’s thicker, which it noticeably is due to the auto dimming ability.

I’d like to take a swing at diagnosing the potential issues myself if possible. I re-tested the next day and again the same but I could feel the passenger mirror was warm as well as the traces on the back window were warm everywhere but that left side, cool to touch. There’s no cuts in any traces that I can see and the window is not tinted, for this reason honestly.

I have some ideas like using a multimeter to see if the mirror is actually getting power but I’m not certain where to even start with the back window? It’s a bummer since the truck is just over 3 years old and I think I just broke 39,000 miles. I’ve got an extended warranty but if it’s something I can learn about I’d like to and I’m not convinced the dealer would be jumping to find a fix either. Only posts I saw were years old with no resolutions. Might need to find cheap thermal camera as well I think that would help.

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[Kanuck]

Being a retired electrician, I would check for power at the windows and check continuity of the heater at both back windows and mirror. Because both don't work, I am suspecting a bad connection or relay that controls that side of the truck. Maybe someone on here can post the electrical drawing for the heaters.

 

[Snakedoctor]

Being a retired electrician, I would check for power at the windows and check continuity of the heater at both back windows and mirror. Because both don't work, I am suspecting a bad connection or relay that controls that side of the truck. Maybe someone on here can post the electrical drawing for the heaters.

See now I’m not sure the mirror is actually not heating. Like I said it’s thicker and even Ford themselves state that the drivers side will take longer to heat up.
I may use this as an excuse to buy a FLIR camera that I can use on my phone. Would be fun and functional.

 

[Snakedoctor]

Being a retired electrician, I would check for power at the windows and check continuity of the heater at both back windows and mirror. Because both don't work, I am suspecting a bad connection or relay that controls that side of the truck. Maybe someone on here can post the electrical drawing for the heaters.

So I’m not sure yet since it’s 86 degrees out and my test was limited but I opened and closed the center window a few times and turned on the heater. The traces felt warm on that back window panel now.

 

[Kanuck]

This is for a 2021 truck...yours should be the same. Because the two systems are different. I would be looking for broken wires or bad heaters.

 

[Snakedoctor]

I guess I was just optimistic. I tried again this morning and again the passenger side and center glass started right away to warm up on the traces and clear the window. Same with the passenger side mirror, both the upper glass and the convex from the tow mirror.

The passenger mirror is only about 8 months old since the original was replaced from an accident. Now I’ll have to do more work to see what’s going on. I could live with the rear window only half clearing but the mirror one I want working properly.

 

[Kanuck]

Because you said accident... something mayhave been shaken up or jarred loose... Also check for loose or discounted plugs.

 

[Snakedoctor]

Because you said accident... something mayhave been shaken up or jarred loose... Also check for loose or discounted plugs.

It was more of side swipe but all on the passenger side. I’ll have to pull the trim off the door and try to check the plug on the mirror. It must not be heating. The passenger mirror was noticeably warm where the drivers mirror you’d have no idea was even heated.

 

[Snakedoctor]

Its frustrating since Ill have to wait to late in the night to even try diagnosing. I tried to do it during the day but its 82 in heavy sun so it feels like 90s. I dont think the mirrors would even try heating up given the temperature.

I have to bring my truck in next week for the rear axle recall work so Ill just bring this up to ford and pay the $100 for my deductible.

 

[Snakedoctor]

On the way to the dealership I turned it on and to my surprise it showed it working. You could see the glass start clearing from the grid lines. On the side mirror you could see the start of where the mirror was heating up.

Fast forward to today another cold morning where I remote started. The whole passenger and center glass was already clear and the rear driver was untouched. I’m now concerned it’s going to be an off and on issue which would make a dealer willing to work harder. I attached a photo of what the window looks like after the rear has been on. You can clearly see the grid lines heating on the good panels.

 

[Snakedoctor]

Dropped it off at the dealership today. I confirmed via infrared thermometer that there’s no heat from the rear grid lines on drivers side rear window. For mirror I can physically feel heat on passenger side but not drivers.

I really wish I knew how this worked or someone knowledgeable could chime in. Is it possible the mirror didn’t work because that side of the glass didn’t? I know older trucks because that’s all I could find on you tube had people bypassing the window grid to maintain heated mirrors.

 

[Snakedoctor]

Maybe someone in the future will need to refer to this thread but dealer called and said there was a module issue/update needed with the HVAC control which was covered under my extended Premiumcare warranty. I do not yet know if this is the final fix though, I sure hope so!

 

[jkosh22]

Hopefully that’s the issue! But just for reference if anyone needs it. I have a wiring diagram for 2021 Lariat, it’s all the same between trim levels, it appears each window has its own grid working independently of the same power and ground. So you most likely have a driver side fridge failure or a loose connection. It it would most likely be the other panels also. Had rear defrost overheat window in my 2013 f150 and blew out the glass, has to replace entire rear window assembly. Hope this helps! Sorry if the photos look bad, I took them off our shop tablet.

 

[Snakedoctor]

Hopefully that’s the issue! But just for reference if anyone needs it. I have a wiring diagram for 2021 Lariat, it’s all the same between trim levels, it appears each window has its own grid working independently of the same power and ground. So you most likely have a driver side fridge failure or a loose connection. It it would most likely be the other panels also. Had rear defrost overheat window in my 2013 f150 and blew out the glass, has to replace entire rear window assembly. Hope this helps! Sorry if the photos look bad, I took them off our shop tablet.

Would the rear window having issue affect the side mirrors as well at all? I know older models if the rear window circuit was broken the mirrors wouldn’t work, at least from what I saw on YouTube.

 

[jkosh22]

No, from what I’m seeing on the diagram is that the DDM (drivers door module) get its power from the BCM, than the DDM send signal to the outer mirror. They don’t have any wires in common. Most likely just a coincidence. Is it a tow mirror or a standard mirror? I traded my standard mirrors for tow mirrors and noticed the tow mirrors defrost a lot slower.

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