Sooner or later your car is likely to have light problems. It could be caused by bad light bulbs, fuses, switches, relays, battery , or simply loose or shorted wires. The light problem might be that one side is on and the other side is off. It could be that lights are always on or won't turn on at all. Or they could be flickering. This video discusses ways to test and eliminate possibilities so that you can quickly focus on the real faulty components. This method can be applied to other electrical circuit, the 6 steps should be:
1. draw a electrical diagram
2. measure every exposed points.
3. mark the measured points on the electrical diagram.
4. construct a truth table (as shown in this video) to eliminate good components and further narrow the search to trace the circuit.
5. once the search is narrowed, remove the faulty component for test.
6. And repeat from #3

Index
0:00 car light overview problems
0:27 typical car light circuit diagram
0:47 problem 1: lights are always on
1:07 brake switch
2:13 three ways to force bulb off
2:55 problem 2: left light off & right light on
3:36 two lights in same bulb
4:30 all lights are off
5:26 other lights circuit diagram
5:46 lights flickering