An open circuit will not operate as the circuit is not complete. Current cannot run from the battery, to and through the component and back again.
To check for an open circuit, connect one lead of a voltmeter to either the negative battery terminal or a known good earth, such as a clean chassis area.
Connect the other lead to a connector in the circuit being tested, preferably nearest to the battery or fuse.
Switch on the circuit, bearing in mind that some circuits are live only when the ignition switch is moved to a particular position.
If voltage is present (indicated by voltmeter) then this section of the circuit between the relevant connector and the battery is problem-free. Continue to check the remainder of the circuit in the same fashion until the problem section is found.
Once there is no voltage present, the problem must lie between the checked point and the previous test point. Most open circuit problems can be traced to a broken, corroded or loose connection.