At least in my '71 the rear pillar light lenses as a whole can act like switches. One end of the lens tabs into the base, the other end can be pulled in or out slightly. I forget which position it is, but one position is slaved to the front light, the other is 'on' all the time. It's sort of subtle and not very intuitive. Sort of like the hidden heater controls?

I don't think the owner's manual even really mentions how they work, but the do show thw 'switch' side of the lens in the page showing how to replace the bulb:

http://www.volvo1800pictures.com/manual/page51.html