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Discussion Starter #1
I know what you're thinking. It's the rear sliding glass to blame, right? Well, I think that's not it. For one, I never use the rear siding window. The problem is the light on the button for the rear defroster/outside mirror defrost doesn't even come on. I assume if the rear window connection was a problem I would still see the button lit up, but that doesn't even light up. According to the manual, 40A fuze un position 32 is for the rear window/outside mirror defroster and also the high power relay in position 4 is the relay associated with this. I checked/swapped the fuse and no luck. Still no light. I also swapped the relay in position 6 (same type of relay, for trailer brake lights or something) and that didn't work either.

Any ideas?
 

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Have you tried sliding your rear window all the way open and ALL the way shut. I say this because one time I didn’t fully close the rear sliding window and it did the same as what you’re saying. The light or nothing would come on from both the switch and the infotainment center. Soon as I shut the window completely everything worked as normal. If this isn’t the case, then this is over my head as I have no idea how ford has this thing wired.
 

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Discussion Starter #3
Thanks. I tried that just a few minutes ago and no luck. Still no light. I assumed if the rear window was to blame I'd still see the light on the button come on, but I could be wrong. You guys that have had that problem would know better than me. Oh well, I'll keep messing with it in my spare time and I hope it's something simple. I don't know if I want to pay for a big repair, but I really do need that for winter.
 

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Discussion Starter #6
Any other ideas or troubleshooting tips? So far I've swapped all the fuses involved (fuses that related to the defroster in the owner's manual) and also swapped the relay in the fuse box. I took the back seat out just to have a look at the wires and everything looks perfect. I NEVER use the rear sliding window because when I bought the truck new in 2014, I had already read about the problems people had with the defroster when they use the rear slider a lot. I did however go ahead and open/close it a bunch of times in case the contacts weren't making good contact. From what I can see, it looks good and I can even still see some of that factory lube they put on the contact area for the rear slider. So, I 'think' it's not the rear window that's the problem.

Just looking for some more ideas that I could try, if any, before I give up and pay the dealer later this year to tell me what the problem is (then I can decide to fix it myself, if I can, or have them do it). It seems like such a trivial thing, but come wintertime, I really need the heated mirrors to work.
 

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Discussion Starter #7
Update

I had been reading where people say the outside mirror heaters will still work even though the rear window defroster and the light to indicate it's on is not lit up. Well, I confirmed today that this is absolutely true in my case and that's good! The mirror heater is what I needed the most anyway. It's about 73 F outside right now and I felt the mirror while driving and it was cool. I pressed the button to turn on the rear window defroster/heated mirrors and sure enough, a minute or two later the mirror was very noticeably warm to the touch. At this point, I'm not going to worry about it since the heated mirrors work. The rear glass hardly ever gets any ice on it anyway.
 

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Discussion Starter #9
Update on this problem. I gave up trying to find a solution myself and was going to live with only the heated mirrors working and no light to indicate it's on. Well, a few days ago on my drive to work (25F), I pressed the button to defrost my mirrors because that still worked. Next thing I know, the light is on and the rear window is showing signs of heating (strips of defrosted areas along the heating elements on the window). Huh? Suddenly it works? I turned it off when I was done and when I got to work, I turned it on again to see if it comes back on consistently. This time the light came on briefly and then went off again. I pressed the button again to turn it off (because it really is on, even though the light is off). I pressed the button again to turn it on, same thing....brief light and then off again. I waited a few more minutes and turned it on again and this time it stayed on (light was on).

I tried that again yesterday too and so far, it seems to turn on briefly and then go off, but if I try a few more times it will come on and stay on (the light - so both the mirrors and the rear window heater are on).

I'm not sure what has changed, but I hope it keeps working.
 
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