It is fairly common in 850 due to the "bad" design of the location of the window switches. Most likely is caused by the spillage of soda/coffee, etc which is sitting on the cupholder above. My left rear power window in my 95 855 had the same problem as yours. Other brickers had tried to open up the window switch module to clean the switch contacts. I had tried that too, but no luck. It was very difficult to open up the module. The common patch is to spray electrical cleaner fluid into the switch from outside, and then execrise the switch to clean the contacts. Give it a try. I tried that trick, but no luck. I then replaced the whole module and everything back to normal. You cannot just replace one switch, it comes in one module.

Ching-Ho Cheng
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1995 855 NA, Bilstein Strut & Shocks, Volvo Protective Grill, K&N Filter, E-Code Lens, Alpine CD-Changer, AutoDim RearView Mirror, 6000K HID