My first response on this thread was due to a duplicate entry by Scott, which has since been changed.
It's amazing what a little detective work can do to solve problems. Scott's original problem was that he couldn't get the engine to continue running after it caught while cranking. While working on his headlight harness, I found no continuity between the I line and the engine gauge feed harness. The copper wires near the 90* connector at the starter solenoid had disintegrated inside the insulation: you couldn't see anything wrong. I replaced that lead and problem solved.
Just last weekend, I had a customer who claimed his ammeter never worked with factory wiring. He cobbled together a system bypassing the headlight/fusebox connector, wiring the ammeter directly to where they start inside the headlight harness. While I was checking out his 3 gauge center dash cluster, the yellow wire for the ammeter had a megaohm of resistance. Closer inspection showed that the factory crimp of the wire onto the pin that goes into the connector was bad; probably that way from the factory. Problem solved.