Try using an alligator clip jumper lead from the ballast resistor output to the coil + terminal, bypassing the existing wire, and take a test drive. That is a quick way to diagnose the circuit, without having to chase down both ends of the wire. It sounds like you've got some corroded terminal connections, or maybe a bad wire. What year is the truck? In the late 1960s and early 70s, Chevy used a long nickel/chrome resistance wire with fiberglass insulation instead of a ballast resistor. That wire is stiff and shiny, and it doubles back on itself inside the wire harness. It's actually about 8 feet long, between the ignition switch and the coil.
Jerry
