Dummy me...I looked all around for a diode or load resistor but fortunately didn't install it...had a eureka moment and figured out it was a mercury switch.
One figures it out that the mercury switch is bad by grounding the lower (visor down) set of contacts and if the lights go on, it's the mercury switch. BTW, I had a vanity mirrow switch, the off/on switch, fail on an 87 760T with 200K+ miles on it, so that's another possibility.

Later models didn't have it, some kind of two wires and a weight or somesuch that one of the techs at the local volvo store said they're dumping in landfills by the dozen (at least his shop is!).

Long and short of it is the tech gave me an old mercury switch from something or other that he had in his 'box O'delight' and I'm installing it in the visor/lighted vanity mirror in lieu of buying the whole furshlugginer vanity mirror/visor assembly at gosh knows what Volvo price.

If anyone cares (since most manuals don't show where the wires go), the vanity light wire pair go down the right A-pillar and are connected to a chassis connector right behind the glove box. The same chassis connector supplies the glovebox light power and ground. I don't know where the ground is for this particular circuit since I never got that far before discovering the failed component that I later figured out was a mercury switch, but I suspect the local ground is that ground plate behind the ECU on the right side passenger kick panel.