After digging around some more last night, I've almost completely ruled out the B wire (-) and the GL/WL wire (+), so it looks like it may end up being the turn signal switch. After removing it from the steering column and looking it over, I think it may be possible to repair it (but I won't know until I dismantle it further and examine what I find).

After accessing the steering column unit (turn signal/light control/wiper control), I did find that the LB wire (for the headlight washer button) was disconnected and hanging free, so I think I'll solder that back up if I otherwise repair my original problem. I don't have headlight washers, but I'm sure I can find a use for it. <img src="/forums/images/graemlins/smile.gif" alt="" />

Thought I'd follow up on this one. It's fixerated now. I found a vampire tap which the previous KHO (knuckle-headed owner) had connected to the WL wire (which becomes the GL wire at the combo light unit in the back) near the connector for the column control unit, up under the dash. The current KHO (me) removed the tap and respliced the wire and now all works fine.