I got it.

Looks like the vehicle was originally equipped with single filament bulbs. Someone who almost knew what he was doing adapted it to two filament bulbs, by running a new conductor: the one which started this thread. Somehow along the way, that patch got undone. The two-filament bulbs got rewired with the original ground as the second filament.

So of course that isn't going to work.

I disconnected the original ground wire from ground, and spliced it into the light switch (brown wire) and eliminated the added conductor.

TA DA