Have you tried supplying 12V directly to the fuel pump? Even though you might measure 12 at the sockets, the wiring might be corroded or damaged so it cant deliver enough corrent to drive the pump. I know, because i changed my pump today, thinking it was knackered.. Only to discover the aforementioned wire issue. As a temp fix i have supplied the pump 12V via the cigarette lighter output -as it is -15 degrees C outside and i`ve had enough for one day..