Solenoid is another name for relay. Electricians have yet another name called "contactor". It would sure help people to understand if everybody used the same name!

Anyway search google.com for how a relay works and that should explain things. Basically a relay is an electrically operated switch. So there would be the two wires you are switching. And then the two wires to power the on/off switch.

Sometimes they use the metal automobile ground as one or two of those wires (- negative). But engines are dirty oily things, maybe they are running wires directly rather than using metal as a ground to be sure there are good connections?

Note the reason they would use a relay is that it is difficult to connect large wires to a key switch. Imagine running those large battery cables under the dash to a key switch!