Car dies in "run" position

Hey Mopar Guru's. This is my first post on this thread. My son has a 74 Plymouth duster w/ a 318. Car starts and runs in start position, but immediately dies when in the run position. We replaced the Ballast Resistor, still had same issue. I bypassed the Ballast by connecting a jump wire, still has same issue. Any ideas on what to try next? should I start checking wires or the ECU, coil??