On the 850/70 cars, the mirror glass can be pried off. It is attached in the back by 3/4 clips that fit in a hole. After the glass is loose, you can unplug the heater element.

The mirror must be turned toward the car, electrically, so you can get some fingers behind it. Use the other hand to catch the glass.

Of course, a dealer can plug in his code reader to see if the temp sensor is working, but in your case I think Holy Water is required :>)

Klaus
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Two good Volvos, a 220 and a V70R