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.

I forgot to say.
Hg gases are dangerous for your health.
To use it under the hood is OK - maybe,
but to use it inside - I don't think thats good.
If the glass-switch breaks, it releases
some gases directly in your nose.
And if you dont find it immediately - for how many years?
That design should get a lemon.
You might survive an accident because of the safety
features - only to get poisoned by the Hg.

my unimportant 2 cent

Thanks for that info.
FYI also the lightswitch under the hood
has a mercury switch inside.
Let me guess 2 ounce of Hg per car
times a half million cars -
or how many were produced -
Volvo-wanna-be-engineers must have bought
a whole year production for that.
Who cares for environment?

Well, the math is probably something like 2gm per car rather than two oz. I forget how many drops per ml, used to be something like 10 drops or so?

The glass capsule has one drop of Hg in it and it's pretty well protected; the mercury switch that came with the visor was metal encapsulated; I doubt that it would rupture in an accident although a fire would probably melt it and disperse the Hg fumes.

I guess I'll know if the glass capsule ruptures if the vanity mirror doesn't light!