Possibly your keyswitch. Get yourself a good multimeter and learn how to use it, it is an invaluable tool for diagnosing electrical problems. With the key in the start position, check to see that the signal wire to the start relay is getting +12V. If it is not then either the keyswitch is no good, or the wiring between the keyswitch and the relay is at fault. Check the keyswitch to see that it is also getting a good +12V feed. I wonder if this clicking noise is coming from a circuit breaker, perhaps there is a short somewhere.