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Always nice when a hunch of mine turns out to be correct, especially when it was such an easy thing to try. If you hang around these forums long enough you pickup stuff like that. I think it was either Spooky Jay, one of the old Steves or Randy who first mentioned that here, probably Spook. Now that you've reminded me, I'll maybe go out and do a proper cleaning of all four of my mirrors. Yes, I would do a pre-emptive cleaning of your other mirror.
As for the rear window defrost, if the light is on in the switch and you don't want to dig into the console then the next stop can be checking for power getting to the rear. Two things you can try and I've done both. I'm mostly familiar with wagon tailgates and it's been a while since I needed to check a sedan defrost grid. It is not uncommon for one or more cross traces of the grid to be damaged and no longer conducting, or else conducting poorly, especially in wagons. If you can steam the glass, even with your breath, then you can visually see which traces are working. Ideal is on a frosty morning and watch it as it warms up or having a crowd of people in the car on a colder, damp day first thing in the morning with the windows closed. Maybe give each an open mug of hot coffee and a newspaper to read with the political headlines of the day to get them really steamed.
It's normally difficult to probe for voltage at the grid or exposed connectors, but well worth a try before needing to remove the side trim to access the connectors for proper testing with a meter. I don't recall that you can probe at the exposed side tabs, leastwise not in wagons.
First you need to find yourself a good chassis ground. The closest screws are usually behind plastic plugs and the screws themselves may be into plastic, not metal, so double check that. The screws holding the door catch are a good ground, but perhaps not so easy to get an alligator clip on and maintain good contact. I forget how far I had to go back to get an alligator clip on a ground point. I may have just taken my 10 foot lead and found something easily used at the rear end or dash
Next is to have a good sharp needle on the end of an alligator clip. Re-sharpen on sandpaper or whatever as needed during testing. Shirt pins easily bend held in pliers, so grab a few pins as spares and grab closer to the tip, careful not to scratch the glass. My preferred weapon of choice is a sturdier T-shaped basting pin stolen out of my wife's sewing kit (I stole two, they now live in my rack of test leads and jumpers).
There's a thick protective clear coat on the grid to prevent damage. They used to have a plastic coating that was more easily probed; the newer seem to be a much harder plastic or ceramic and much more difficult to probe. Probe hard directly into the wider traces on the side near the connector tabs, not the fine traces unless you really need to, wiggling the tip, not scratching. One side will have voltage and the other will be ground, checked by the Ohms function -either figure out which side by trial and error or know that in 700/900s it's normally ground on the right passenger side. If the meter flickers then you've got something and can consider that side of the grid ok.
What often happens is one or more thin traces are either damaged and broken or no longer conduct enough current. If you can visibly see a scratch or damage then that's a potential break. To find and confirm breaks you can use a sharp pin and the Ohms function. The traces are parallel so voltage runs around the break. Resistance to ground will be greater when your probe is on the powered side of a suspected broken trace compared to the same spot on a good trace and you can easily confirm the break by moving the probe side to side of the break. I usually narrow down breaks using two needles working inward either either side of the thin traces rather than using chassis for ground.
Now having said all that, these newer thin traces are very difficult to probe and you risk damaging a thin trace. My preference now is the steamy window approach. If nothing works then it's a either a broken trace at a window side tab connector or a wiring problem to a connector needing to have trim removed to isolate.
There are window defroster grid repair kits with a small bottle of copper metallic paint that you can get at many good auto supply stores. You bridge the gap with paint and let it cure. The problem is you need to scrape the clear coat off, exposing a nice bare original metallic surface, while not scraping it off the glass. I've used these a couple of times with very limited success, and even then it was on the older wider thin traces, not the newer more invisible very thin traces behind a harder coating. I no longer bother trying to repair them as long as a few traces are working -the window will eventually warm and clear. If I ever have a problem near a side connector then I'll try again and research the best repair kits.
Hope that helps.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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