A continuity check is necessary to determine if there is a break in the circuit. Remove voltage from the circuit and test the circuit for continuity with a light bulb connected to the power supply.
Connect the light bulb wires to the ends of the circuit (or to the positive end and (mass) car). If the control lamp lights up, there are no breaks in the circuit. If the light does not come on, then there is an open somewhere in the circuit.
Similarly, you can check the switch by connecting a test light to the switch terminals. After turning on the switch, the control lamp should light up.