To start with, remove the starter relay and use a test light to check for voltage on the terminals in the socket. One must have 12 volts all the time and one must have 12 volts when the ignition switch is turned to the "crank" position. If you have both of those, move the test light's ground clip to the battery positive post so you can test for grounds. You should find continuity to ground on one of the remaining two terminals. The fourth one used to be real easy to test; it went through the neutral safety switch to ground. Unfortunately the engineers saw fit to involve a computer in that circuit now to replace the uncommonly reliable switch. If the first three circuits appear to be working, I'll try to figure out which circuit to go to next.