* Bulbs: The most obvious cause. Check both high and low beam bulbs. They might be burnt out or loose in their sockets. Try replacing them with new ones. Make sure you get the correct wattage.
* Fuses: Even if other things work, a specific fuse for the headlights might be blown. Check your fuse box (usually under the dash or in the engine compartment) for a fuse labeled "headlights," "headlamp," or something similar. Replace any blown fuses with the correct amperage.
* Headlight Switch: The switch itself could be faulty. It might be internally broken or have a loose connection. Testing this usually requires some electrical knowledge and a multimeter to check for continuity.
* Wiring: A break or short in the wiring harness leading to the headlights is a possibility. This is often difficult to diagnose and may require tracing the wires from the switch to the headlights, looking for damage, corrosion, or loose connections. This often involves removing some interior panels or possibly some parts of the engine bay.
* Headlight Relay (if equipped): Some 66 Mustangs might have a relay in the headlight circuit. A faulty relay can prevent the headlights from working. Replace it if you suspect this.
* Grounding: A poor ground connection can prevent the headlights from working, even if the power supply is fine. Check the ground wires connected to the headlight assemblies and the chassis. Clean any corrosion or tighten loose connections.
* Multi-Function Switch (if equipped): This switch controls various functions like turn signals, wipers, and possibly the headlights. A fault in this switch may affect the headlights specifically.
* Dimmer Switch: If your dimmer switch is malfunctioning, it might prevent the headlights from working.
Troubleshooting Steps:
1. Start with the simplest: Check the bulbs and fuses first. This is the quickest and easiest check.
2. Visual Inspection: Carefully examine the wiring harness for any obvious damage or corrosion, especially near the headlight assemblies and the switch.
3. Use a multimeter (if you have one): Test the voltage at the headlight sockets with the lights switched on. If you have voltage, the problem is likely with the bulbs or their connections. If no voltage, the problem is further back in the circuit.
4. Consult a wiring diagram: A wiring diagram specific to your 1966 Mustang will be incredibly helpful in tracing the circuit. You can usually find these online or in a repair manual.
If you're not comfortable working with car electrical systems, it's best to take your Mustang to a qualified mechanic. Incorrect wiring can lead to further damage or even fire.
