An isolator switch usually goes into the cable which goes from the solenoid to the positive terminal on the cranking battery.
However, I'm not the best qualified person to advise you on boat electrics.
Even after having completely rewired two boats I'm still rather baffled, but they both worked so I suppose I must have done it right.
I do have an excellent book on board my boat, which has loads of excellent diagrams, unfortunately I can't remember it's title, I will dig it out next time I'm on board and let you know.
I hope this helps.
Mike
PS. I tried to email you but the address you gave me didn't work.