I agree with taclem based on first hand experience....
My grandfather had a '72 C20 that my dad had given to him. (Lived in Ohio back then and the road salt ate the body off this truck.)
When the turn signal was on all the lights (tail lamps and side marker lights) would flash, not real bright, but they would flash.
I took a wire, stripped the insulations off, and touched the bumper with the one end and the tailight housing with the other end. Viola! that fixed it.
We took each of the tail lamp assemblies off and hard wired a pigtail from the housing to the trucks frame, aka made our own ground straps.
It fixed the problem.
Since then whenever I have an electrical glitch / gremlin the first thing I do is run a temporary ground wire to see if it fixes the problem.
John
