Your 1996 GMC Sonoma does not use an "auto switch" for the fuel pump. The relay is controlled by the powertrain control module. Power comes from the ECM/BATT fuse in the instrument panel fuse box. Power is supplied from the fuse to the fuel pump relay in the relay center (not sure where that is). When the PCM commands the relay on, power is sent from the relay to the fuel pump. You will need to diagnose why no power is getting to the fuel pump.