Strange fuel pump electrical problem

2001 Jimmy. Since the test drive, the fuel gauge would intermittently zero, even with as much as 3/4 of a tank of gas. We thought it was a problem with the sending unit; so we replaced the pump assembly. Fired up the vehicle, it ran fine until it reached operating temp. Then it stopped. Now we find that the bus that powers the fuel pump relay is getting absolutely no voltage. No idea why, especially since the old fuel pump was getting power from somewhere before. But to power the pump we have to bypass the ECM and apply 12V directly to the bus. Now, here's the weird part: The original fuel pump has worked, off that same bus, for the last seven months. It looks like I have to replace the fuse/relay block: taking it apart to trace circuits revealed some strange deposits, apparently salt corrosion, at the fuse socket layer. But why would the 12V bus suddenly lose power; or how would the fuel pump be getting power if there was no voltage on the bus?

Corrosion can occur where the bus leaves bolt together. The fuel pump will also receive power from the oil pressure switch once the engine has developed 7 PSI.