Too late to save Dr. Chip some money, but the usual failure mode of these small, high current switches is burned contacts. Once you have the switch out, it's not hard to get it apart (carefully!...old plastic) and clean up the copper contacts with an abrasive like very fine sandpaper, emery paper, or even a pencil eraser. It only takes a bit of pitting/burning on these little rascals to render them intermittent, then dead. (Ditto for seat heater, power sunroof switches, etc.)

The gent who sold me my S90 was very honest in disclosing its problems and a non-functioning rear window when operated from the driver's door was one. Since it worked from its own door switch I knew the expensive parts were OK and it was just a switch issue. The above fix worked fine.
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Bob (son's 81-244GL B21F/M46; dtr's 94-940 B230FD; my 83-244DL B23F/M46, 89-745(LT1 V8), and 98-S90; and four old motorcycles)