Connect one of the test light probes to the negative terminal of the battery or (mass) car. Connect the second probe to a connection in the circuit under test, preferably as close as possible to the battery or fuse.
If the control lamp lights up, voltage is present. This means that the part of the circuit between the connection and the battery is good.
Continue checking the rest of the chain in the same way.
When you find the point where there is no voltage, it means that the source of the fault lies between that point and the last point where voltage was present. Most problems are caused by a bad connection.
Attention! Do not forget that in some circuits there is voltage only if you turn the key in the ignition switch to a certain position.