"Turned out to be the glove box door, which was not closing far enough to depress the light switch plunger."

This is a very common 850 problem. It happens because the door is plastic, the latch is in the center and the switch is at the side. Thus the switch is putting a constanting bending pressure onto the plastic door, and over time the door bends enough that the switch no longer closes. Our 850 was dead upon our return to the airport parking lot once because of this problem. I fixed it by epoxying a nickel into position on the glove box door to act as a striker plate for the switch. Five years later the fix is working fine. I like to say that this is only time I ever fixed our Volvo for a nickel!

This is the kind of engineering mistake Volvo should not be making.

John