Safety First: Disconnect the negative battery terminal before starting any wiring work. This prevents accidental shorts and shocks.
Understanding the System:
* Headlight Switch: This is the main control. It'll likely be a multi-position switch controlling parking lights, low beams, high beams, and potentially other functions.
* Headlight Relay(s): 1978 Cadillacs likely use relays to switch the high current for the headlights. This protects the switch from overheating. Finding and understanding the relay circuit is crucial.
* Wiring Harness: The main wiring harness carries power to the switch and relays. Look for color-coded wires. You'll likely need a wiring diagram specific to your Cadillac model.
* Headlight Bulbs: These are usually H6024 or similar (check your owner's manual or the bulbs themselves). They have separate filaments for high and low beams.
* Grounding: Good grounding is critical. Ensure the headlight housings are properly grounded to the chassis.
Steps (General Outline – Requires a Wiring Diagram):
1. Obtain a Wiring Diagram: This is the most crucial step. You can find wiring diagrams online through forums dedicated to classic Cadillacs, or possibly through a repair manual specific to your 1978 Cadillac model. A Haynes or Chilton manual would be helpful.
2. Inspect Existing Wiring: Carefully examine the existing wiring harness for any damage, corrosion, or broken wires. Repair or replace damaged sections as needed.
3. Trace the Wires: Follow the wires from the headlight switch, through any relays, and to the headlight assemblies. Note the color codes and their functions.
4. Identify Relays: Locate the headlight relays. They are usually small boxes with multiple wires connected. If a relay is faulty, replace it.
5. Connect to Headlight Bulbs: Connect the correct wires to the headlight bulbs based on your wiring diagram. Pay close attention to high beam and low beam circuits.
6. Grounding: Ensure the headlight housings are properly grounded to the chassis. Poor grounding can lead to dim headlights or non-functioning lights.
7. Test: Reconnect the battery and test the headlights in all positions (parking lights, low beams, high beams).
Without a wiring diagram, working on the electrical system is extremely risky. You could easily cause damage to your car's electrical system or even start a fire. Investing in a service manual specific to your Cadillac model is highly recommended. A professional automotive electrician is always a safer option if you're not comfortable working with car electrical systems.
