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="" />