my understanding is that many times the small wiring to and from the helm creates such a voltage drop that the average starter solenoid made for auto applications will not fire without the slave relay that is designed to fire on lower voltage. Years ago on our old jet boat we had to run a ford solenoid to fire the gm solenoid because of the long wire run. We were told to use ford solenoids as the slave as they would fire on less voltage. Don't know if it was all true but it did work.

It been my experience that the stater assist relay willl die long before the starter solenoid. To test, simply use a volt meter. There should be only 4 leads: a ground, a solenoid line from the ignition switch (both small gauge) a 12 volt supply and a lead to the starter solenoid. Put the VM between ground and the solenoid line and turn the key to start: you should go from 0 to 12volts. Now verify that one of the big leads has 12 v when the key is on, then move the VM to the other big lead and turn the key to start; you should go from 0 to 12v. If not, the relay is bad. At least that how my Merc works.

JT's on the right track with the length of wire from helm to selenoid(relay).In your car the ignition switch is only 5-6 feet from the starter selenoid and battery.In a boat the power has to go from the batteries to the helm ignition switch then back to the motor.Thats about 40 feet of wire.It would have to be pretty heavy wire to engage the starter selenoid and even then voltage drop would be an issue.To test just jump it with a hot wire from the battery.The selenoid will have a hot wire,a trigger wire from ignition,and a wire to starter selenoid.If you jump the trigger wire and it cranks your problem is up river.If it doesn't crank but does when you jump the wire to stater then it's the selenoid(relay)If it still doesn't crank jump the selenoid right at the starter.If it cranks theres a problem in the wire from the relay selenoid to the starter selenoid.If it doesn't crank there is a problem with the starter selenoid,starter,or wiring to starter.You can here if the selenoid is engaging when you jump it.Don't be quck to pull the starter as the problem is most often wiring related.Dirty terminals,rotten wire end,bad battery swich,etc.I have been guilty of looking for a big problem when it has been as simple as a rotten wire end.I pulled a starter a year ago thinking it was bad because it would not crank on either battery and it turned out to be a bad ground wire.The grounds were common from the batteries but I just wasn't thinking out side the box till I stood on my head for 4 hours changing a stater.