Have you tried a temporary hot wire directly to the upstream side of the coil to see if you can isolate the open circuit to a specific area? It's about 5 minutes' worth of troubleshooting with a piece of primary wire that will narrow down the possibilities of where to look for the problem.
A 5 gallon plastic bucket, a beer can, a piece of stiff wire, and some peanut butter makes a self-resetting mousetrap that catches them by the dozen.
Jerry
