In a bid to help out until John provides the answer... (-: I think your thinking in type...Is perhaps not far off in that as best I understand it the ign light is made live from the ign switch when switched on, the ign light picking it`s earth up at the alternator. As you suggest once the alternator is spinning the terminal which was once supplying the ign light with an earth becomes live (hot) thus the ign light deprived of an earth no longer illuminates.
Again it seems you may be correct in thinking that with the alternator now making the former earth that it supplied live (hot) the circuit is acting in a reverse direction with the ign light perhaps finding a route to earth through some other of a few ignition switched circuits..?
I guess there may be only one wire (White..?) at the ignition switch..? Which may then go off to the fuse box to supply all ign switched kit, a partial earth being found through any one of the circuits there, that switching the wipers on apparently increases the earth potential at the ign lamp is intriguing indeed, and might cause me to disconnect the wire providing power to the wiper switch, then fire up the engine to check the difference in any on the ign light...
I do remember stripping my B`s switches and cleaning the internal contacts which were pretty well fouled up with verdigris, the hazard switch in particular caused all sorts of strange anomolies which disappeared once cleaned... As best I can remember the switches could be opened up by moving the rocker which pivots on two pins which fit into holes on either side of the switch casing. Deflecting the side of the rocker enough to disengage the pin will enable the rocker to be removed... and the internal spring to fly of into the distance never to be seen again...though as luck would have it... that did not happen when I stripped my switches..
Hopefully by the time I have finished typing this John will have provided the answer...(-:
