I am sitting here checking out a mercruiser wiring diagram and if your system is merc and original there sould be a yellow wire going out from the key switch through the enginr harness connector to the engage terminal on the slave solenoid and the slave solenoid should have the battery pos. cable from the circuit breaker connected to one side and from that terminal to the large start terminal on the starter and the other side of the slave solenoid should have a yellow wire that should be connected to the starter solenoid terminal that is mounted on the starter basically the slave is just a relay for the starter solenoid engage signal. Also noticed that the electric choke gets voltage from the yellow wire between the slave solenoid and the starter solinoid.

Scott