The starter relay gets it power from the key via the start switch, and the start relay grounds thru the charge relay. This is done to prevent the starter from being accidentally engaged with the engine running provided charging system is working. I just looked in my Series II service manual. My guess would have to be your relay is somehow powering up, either from internal fault, or possible you have a harness issue and some power is being cross fed from another source.
Is there a actual wiring diagram in your owners manual? They are there for the Series III. I got a copy shop in town to blow up all three of them for mine, engine compartment, cab and chassis, and had them laminated like I do for all my stuff, sure come in handy when working on them. The ones for Series II are in the service book if you don't have one.
Camera on my phone is all scratched up so I can't take decent pics anymore and I don't have a scanner either. But the diagram they show for how that charge relay is tied into the start relay is pretty straight forward and you should be able to trace it back without a schematic. If you do need more specific info, ask, have to find some batteries for my old digital camera and go that route maybe lol.
Contracts are made to be broken, a handshake is the law of God.