Yay! Bad ground's the culprit.
I had replaced the AD single-filament park light bulbs with dual filaments in new sockets. The right indicator worked, left didn't. I crawled under the left fender with the tire off and scraped paint for a good metal to metal contact. Still no signal. But the left park lamp lens was loose even though the bezel screw bottomed out. So when I shimmed the gasket to tighten the lens, I saw that the socket and reflector sat in a primed and painted bucket. Scraping paint where the socket touched fixed the problem. Now the truck has turn lights and four-way flashers!
For the brake and tail lights I plan to run a ground wire from the bulb mounting brackets to the frame.
Thanks the info.
