Lots of unknowns here.
I initially thought you had a electric motor there based on:
Electric project section.
Diagram showing 12v-, that's more typical to some sort of polarity switching of a electric motor. Think the button on hand drills to change directions.
"To starting motors", suggests multiple motors
Later on I see different.
So:
1. The 12v-. Are we talking a simple ground?
If so, that would be designated like so, upside down Pine tree or spelled gnd:
l
l
_____
____
_
2. The device at the top right, is that the relay or the solenoid (a bigger relay) ?
If relay, then its output becomes the solenoid control input (the small red wire)
The relay output becomes the solenoid input.
? why is the relay needed? some interlock circuit?
3. The relay is two circuits, a control circuit that switches the relay on/off
And the switched circuit, what the relay is controlling (the solenoid)
You have to verify hot and ground for the control circuit and input/output for the switched circuit.
The diagram is missing the 12v+ switched current input.
Likely that the main switch output should have a heavy wire going to that relay.
And then you have what appears to be a built wire harness.
Hard to say what has been done for you and what you still need to do.
Can you incorporate that harness into your diagram, making it clear what is wire harness?
4. The white wire? That came out of nowhere. What's it for?