No spark 1973 360

So I went to start the Dart Sport tonight to go to a car show and would not start. No spark confirmed and the FBO was not pulsing red while cranking just solid red. Unplugged/reseated the the ECU harness at the pickup at distributor and it fired right up. I had a similar issue like 5 years ago and after testing all the connections and the pickup then it had started with an different stock ECU and then was fine with the FBO ECU since.
Wonder where to go with this? Wiring harness replacement from distributor to ECU or the distributor pickup coil?