1. Check the Bulbs: This seems obvious, but it's the easiest first step. Replace all the bulbs (tail lights, side markers, and license plate light) with new ones of the correct wattage. Sometimes a bulb can appear fine but be intermittently failing or have a weak filament.
2. Grounding Issues: These older cars can suffer from poor grounding. Check the ground wire connections at the rear of the car. These are often bolted to the chassis. Clean them thoroughly with a wire brush and ensure a good, tight connection. Corrosion can dramatically affect electrical conductivity.
3. Wiring Harness: The most likely culprit is a break or short in the wiring harness running to the rear lights. This is often damaged by age, wear and tear, or rodent activity. You'll need to carefully inspect the harness, starting at the fuse box and tracing the wires to the rear lights. Look for any visible damage, breaks, or corrosion. This is time-consuming and requires some electrical knowledge.
4. Multi-Function Switch (Turn Signal Switch): In some vehicles, the multi-function switch controls the running lights, turn signals, and hazard lights. If this is faulty, it can affect all your rear lights.
5. Tail Light Assembly: There's a possibility that the issue is within the tail light assemblies themselves. Check for any loose connections within the housing.
6. Relay: Although less common, there might be a relay controlling the rear lighting circuit. Locate and check the relay (consult your vehicle's wiring diagram). You may be able to swap it with a known good relay to test it.
Tools and Resources You Might Need:
* Test Light or Multimeter: This will help you trace the circuit and test for power and continuity.
* Wiring Diagram: A wiring diagram specific to your 1985 Oldsmobile Cutlass Supreme is crucial. You can usually find these online through forums dedicated to classic car repair or via a Haynes/Chilton repair manual.
* Patience: Troubleshooting electrical problems takes time.
Important Safety Note: Disconnect the negative battery terminal before working on any car's electrical system to prevent shorts and shocks.
If you're not comfortable working with car electrical systems, it's best to take your car to a qualified mechanic. The problem could be something simple, but could also require significant repair work.
