No power at all

The box/ ECU tinwork MUST be grounded. That is the return path for the circuitry. I would scour some paint off the backside of the box, around the screw holes, maybe use some anti-ox electrical grease, and use bolts. Maybe put the nut in the engine bay so it doesn't corrode from road splash. Use star lock washers

The same is true, by the way, of the voltage regulator