I am a new owner and I am unable to determine the function of a toggle switch located under the driver' dash adjacent to a push pull vent handle. It appears to have 3 selected positions but nothing happens when either are selected with the ignition key on or off. My manual does not describe its function other than referring to it as "lighting". A similar 2 position switch located on the passenger side is identified as a "map" light and in turn works turning on a close proximity bulb. There is no similar bulb or socket on the drivers side.

As you're new to the 1800s, be sure to bookmark the link Ron gave you to his great Sw-Em site where you'll also find links to other great stuff. There's also a whole bunch of 1800 stuff, including green manuals, on the UK Volvo Club (membership required to access some of it) and on the Australian Volvo Club in their archive ozvolvo.org/archive (files are not indexed, many file names are rather obscure so you'll want to spend some time digging around). Remember to check out the 140 stuff as some of the 1800 stuff is often incorporated in the same manuals.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Also http://volvo1800pictures.com/
All of the manuals and parts books there to view and download if you wish. Production details, Brochures etc. World wide register for 1800's. If yours isn't listed Karl-Erik will be pleased to add your car along with a few photos.

On my ‘73 ES that switch dims the OD indicator light and one other indicator light - I can’t recall which one - maybe the high beam indicator. Those lights can be annoyingly bright on a dark night, especially compared to the weak instrument lighting, lol. The switch has two positions. One position sends full current to the indicators and the other position limits the current.

Chris;

The switch you refer to (Item 95 on this Wiring Diagram for '73 ES: https://www.sw-em.com/1800ES_73_Wiring_Diagram.jpg also described on page 16 of Operating Manual for ES: http://volvo1800pictures.com/sweden/Volvo_1800_dokumentation_handbooks_en.php ) is a TWO position switch which indeed dims OD and Hi-Beam Indicators.

...but in the earlier configuration for '70E, there is a THREE position switch in that same location...it is the Courtesy Light Switch with function as I describe.

This early/late configuration change can be somewhat confusing, because the switch at that same location have a completely different function from early to late production! ...Gregory clearly has the early configuration as shown in this Wiring Diagram: https://www.sw-em.com/1800E_71_Wiring_Dagram.jpg and as referenced in the previously linked Tech Notes (I couldn't find an early Owners Manual to reference).

Cheers

Gregory;

Dave has it right...that Switch on drivers side vent panel selects (Drivers side only!) either ON, OR Door controlled, and that Switch on Pass side vent panel controls Map Light (only). Both Light fixture have local controls also.

See also: https://www.sw-em.com/Courtesy_Light_1800_Notes.htm

Cheers

One would assume that's the dome light with the three positions On, Off, and Door Switched. Check all fuses, it will likely be one of the 8 amp ones. If that checks out then pull the dome light bulb to see if it's blown (use a meter). If the bulb is okay then you will need to start tracing power to the dome light, then to the toggle switch then to chassis ground or the door switch, depending on switch position. Corrosion at the door switch is often a source of problems if it's only not working in Door Switched mode, just try to be careful you don't arc the switch contact to chassis metal when removing the switch.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now