Re: Car doesn't start?
Left side of diagram is the power (100A+) circuit, right side is the switch.
If it were me, first thing I'd do is listen to it. If the click is solenoidish, I connect a voltmeter to the "S" terminal on the solenoid (pin 50 on the diagram) and see if 12v is getting there when you turn the key (stick or automagic ?)
If it is, the problem is in the power side (left) down to 30 on the solenoid, if not, it is on the right.
From here I'd just do normal troubleshooting with a meter and a 12v lead. No big but might take a little time depending on how accessible the connections are. No Big.
Are things like MAF and CPS orABS issues I'd pull out an O'scope and look at signals but this is simple on or off.