Doesn't get any simpler than that.
But NOT to the f*c*ing starter relay.
The starter relay is triggered by the AUX switched voltage.
Turn on the ignition switch. If the source lead to the AUX relay switch is low, then the ignition switch or related wiring needs tending.
Else ...
Trigger the AUX relay and measure the other side of the switch. It should go from 0 to good. If it's low (and the ignition side is good) , then the relay is bad.
If you press the starter button, and the relay doesn't trigger, then the circuit isn't grounding well. Does pin 17 show continuity to ground? If so, check the wire between the relay and the ECU.
If not, then you have to look at the override switch, gear switch, and anything else the ECU might not like for starter operation.
And as always, check to make sure all ground connections are clean and tight.