Is this automatic or stick. If automatic, the neutral safety switch down on the left side of the transmission GROUNDS in park or neutral. This is connected to one of the small push-on "flag" terminals on the relay, which is the coil. The other flag leads (usually yellow) through the firewall to the ignition switch "start" terminal. Very very simple circuit. I would inspect the key switch and connector / terminals, and inspect the bulkhead connector, AFTER checking the NSS