what i found, is the wire develops a resistance in the soldered junctions at the tail lights. this causes the light to not blink. turn the signal on and trace it with a light check the junctions at the rear it is connections that were factory solder (spelling is wrong) is. it usually gets corrosion and causes it not to work. the best tool is use the light starting at the fuse box.

68 400 h.o 4 spd to go.