Dash Oil and Brake Warning light - how are they triggered in the start function?

For sake of clarity in this discussion, now we know what you mean by grounds.
I don't call those grounds. To me they are a connecting wire and a ground wire is normally at zero voltage.
While I have in the past described that type of connecting wire as a return or ground when explaining the dome light switches, I refrain from that if I can.
The wires to the sending units are at system voltage when the key is in run and the switch is open. The wires between the dome light and the switches are hot all the time.

Is it important? IDK but it could be. Its kindof like a switch leg in residential wiring. Switch is off but the wire leading to it is hot and so is the fixture.