To find where things go bad, start at the battery and measure voltages as you move along the circuit that supplies power to the device that is not working. Assuming no voltage at the device and good voltage at the battery, find some point in between and measure there. If voltage is good, then move towards the device and repeat, otherwise move towards the battery and repeat the test. The point of failure will lie between the point where the voltage is good and where it is bad. Keep dividing that segment of the circuit in parts until you find where the fault is.