A lighted 12v single pole switch, such as a rocker type commonly used in cars, will have 3 terminals on it. One is a ground, one is 12v from the power supply or battery, and one is connected to the load, and has the 12v when the switch is closed, or "on".
The ground terminal supplies the return path for the tiny light bulb inside the switch. The other side of the light bulb is connected internally to the load terminal of the switch, so that when the switch is closed, both the load and the light bulb in the switch turn on.
The ground terminal supplies the return path for the tiny light bulb inside the switch. The other side of the light bulb is connected internally to the load terminal of the switch, so that when the switch is closed, both the load and the light bulb in the switch turn on.