As long as we are talking about the inertia switch, it is a primary safety item. In addition to shutting off the fuel pump in the event of a major deceleration crash, it also unlocks the doors if your door lock circuit is hooked up and working. Make sure you have complied with SC-06-7/82 dealing with relocating the switch and installing the updated switch with a white splotch of paint. On a lot of Deloreans there were problems with the fuel pump electrical circuit. Some of the most "creative" wiring solutions seem to get employed by PO's to get the fuel pump to run. Clean the mess up and make sure it works by actually testing both the fuel pump circuit AND the door lock circuit. As for the ground, it is one of the many deficiencies in the electrical system, not having a good grounding system and undersizing the wiring. When the fuel pump starts to go bad it overloads the circuit on both sides, the feed AND the ground.