Ford computer only runs the fuel pump for 2 seconds when key is turned on, it won't start again until engine does, so pain to test voltages that way.
Find the self test connector in the engine compartment, by the firewall drivers side or passenger side
Picture here: Ford Ranger - Testing EEC-IV Equipped Engines
You will see the Fuel Pump slot labelled
That is the ground for the fuel pump relay, so if the key is on and you use a short jumper wire to ground that slot the fuel pump relay with click closed and pass power to the fuel pump.
If fuel pump comes on then computer could be the problem
With that jumper wire in place you can now start to check voltages at the inertia switch, both wires should have 12volts.
And at the fuel pump.
Relay with Green base is fuel pump relay, Brown base is EEC relay, EEC = computer, this relay also powers injectors and ignition
3rd relay would be there if you had air conditioning