Modern ECMs run low voltage circuits, you have assumed it's a ground, and the lights lighting up could be, but that doesn't mean your ECM positive circuits are right either.
Also, eliminating the backfeed doesn't fix the lack of a good circuit for the ECM or the pump. At this point, the entire wiring harness would be suspect, and especially those areas up front, or around the tank if it took a good shot there. The first start would be resetting ever plug and finding every ground using a wiring diagram
This is one of those jobs nobody likes to take, because it could as easily be a plug under the dash as a ground on the radiator support.
That being said, you should be able to test each leg of the relay circuit, then potentially jump that circuit. If the pump stays on, then it is either the trigger circuit to the relay, or a ground. If it doesn't pump, then it is likely post relay.
However, my hunch is that a plug somewhere is damaged, loose, or corroded causing the U1000. Often there are multiple CAN bus circuits in a vehicle and the harness can run anywhere. Recommend if you don't have one, you get a wiring diagram and look under the carpet, dash, and anywhere the harness will run
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 445 cid FE, headers, RPM intake, 1000 HP series Holley, 4 speed