I would use a multimeter to trace where the power stops in the circuit, but if you want a quick relay test then on the diagram, swap the fuel pump relay with the Wide Open Throttle A/C Cutoff Relay, assuming they are the same type as shown on the diagram.
If the pump then works, just as a sanity check (in case the contacts were corroded or something and merely unplugging and replugging them got it working) swap them back and see if the pump works.
Also check fuse 12 in that same box, either with a multimeter, visually, or swapping it with the headlight fuse #11 right across from it.
The inertia switch is probably behind the trim panel on the right in the passenger side footwell.
While it could trip or fail completely, I'd sooner think it's a fuse, relay, or wiring/connector problem unless the vehicle was in an accident. With a multimeter you can tell whether power is getting to and past the fuse and relay, and whether that switch is conducting. If you measure 12V at the output from the inertia switch then there's a problem between it and the pump.
Where did you tap into the wiring to the pump to connect it to the battery?