Thanks for the suggestion. I use the side of the socket as ground source. The ground is common for both the tail light contact and the turn signal contact. Since the ground works for the tail light, I assume it's good for the turn signal filament as well (dual filament bulb). Also, when I put the bulb in backwards (with a little fiddling), so that the lail light contact touches the socket's turn signal contact, then the turn signal filament lights up. I think I must be missing something basic. I'll try your suggestion about checking resistance ground to chassis and see if that produces any enlightenment (no pun intended). Thanks much.