Phil;

That lift-lens-to-turn-on-lamp is indeed on of the ways the fixture can be supplied ground, but it is ONLY supplied to that fixture and so only turns on that lamp. If you pop off the lens, the manner in which it is intended to work is quite novel and unique, but unfortunately those fixtures are also some of the Lucas crap components...to go along with the (unsoldered) bullet connectors, and finally that under-dash switch you mentioned.

Bob;

ALL of these components are suspect until checked out. I just checked the factory Manual and wiring diagram for a '71 E. There is NO Switch operation and functional description in the text (there may be one in Owners Manual which I dont have a copy of), but from the wiring diagram (there are some interesting, and strange quirks!), I am able to deduce the following:

1. Power is supplied from Fuse 10 by way of a RED wire to the DR side fixture (second RED wire at DR side supplies power to Pass side).
2. Both fixtures have the "local control" capability as described above, at any time, but when the control switch is is set to the ON or "Door-activated" position, it controls the DR side fixture ONLY (strange as this may seem...this info was gleamed directly from a wiring diag...owners of one of these models may want to confirm this...perhaps it is an error on the wiring diag...it wouldn't be the first time...I've seen the dist rotation shown as CW too!).
3. The PASS door ALWAYS controls the PASS side fixture (irrespective of switch position!)

If you like, you can look at this same wiring diag and sanity check me!
See: http://www.intelab.com/swem/1800E%20Wiring%20Diagram.jpg
[For reference, items 40 are the courtesy light fixtures (each, with a local switch to ground), item 47 is Pass side door sw, item 64 is Dr side door sw, item 15 is courtesy lighting control sw]

Map reading light is supplied by Fuse 10 also, but otherwise completely seperately controlled with its own switch.

Hope that helps