Ground. Ground. Ground. Seems like this is asked every week. When some bulb other than the proper one is flashing that means the current is flowing across the turn signal filament, back across the park light (or tail light) filament trying to find a place to ground itself. In Slow..Rider's case it was finding it at the instrument panel. In other cases it travels the park light wire to the other side, goes through the other parklight filament and finds a ground there.

You guys with turn signals in the hood might consider a ground strap from hood to cab. I don't know but I'm guessing they're supposed to ground through the hinges but don't do it after 40 years. That would explain Slow..Rider's working with the hood closed but not open. It's finding a ground at the latch.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.