86 Ford , 302. Where does the signal come from that tells the fuel pump relay to send power to the fuel pump?
The ECM. But, it goes through an ‘impact detector’ that is like a circuit breaker, only a sharp hit will make it pop. This is a small black box behind the seats that has a white reset button on top.
Initially from the battery, through the ignition switch and the emergency cut-off switch (also known as the inertia switch, meant to shut off fuel flow in case of a crash). There is a fuse in the circuit, somewhere, too.
Start at the beginning, and follow the circuit. A service manual (Haynes or Ford factory) would probably help quite a bit. Try Books4Cars.com for an inexpensive used manual.