A vehicle's headlight switch is a relay that controls not only the headlights, but high beams and dash lights, too. Some signs that your vehicle's headlight switch may need replacing include non-functioning headlights or high beams.
Pull your vehicle into a dimly lit area, such as a garage. Turn the ignition key and flip the vehicle's headlight switch to the "on" position. Have an assistant stand in front of your vehicle as you do this. Check first to see if either headlight is working. If only one is burnt out, it is likely you may only need to change a bulb. If neither come on, you may need to replace the switch.
Turn on the vehicle's high beams. Have an assistant see if they come on. If neither come on, and the vehicle's low beams do not come on, the headlight switch may need replacing.
Check the parking lights. Again, if they do not turn on, this indicates the headlight switch needs replacing.
Access the vehicle's headlight switch. On some vehicles, the switch may be mounted in the dash, and will requiring removing a section of the dash with a screwdriver. If the switch is located in the steering wheel column, you may have to remove the bottom of it with a screwdriver.
Connect the positive (red) lead of the test light to the H terminal on the headlight switch, and negative lead to a metal ground. The H terminal will be clearly marked on the switch. If the test light does not turn, on the switch needs replacing.